The phrase to put one’s feet under the table, and its variants, mean:
– (literally) to sit or settle at a table;
– (figuratively) to make oneself at home; to establish oneself securely and comfortably in a situation.

The phrase to put one’s feet under the same table with somebody, and its variants, mean:
– (literally) to sit at a table with somebody;
– (figuratively) to associate oneself with somebody.

The earliest occurrences that I have found of the phrases to put one’s feet under the table and to put one’s feet under the same table with somebody, and of their variants, are as follows, in chronological order:

1-: From Private Life of Napoleon Buonaparte, published in The British Monitor (London, England) of Sunday 7^th November 1819—in the following, Napoléon Bonaparte (1769-1821), the First Consul of the French Republic, talks to the narrator during the passage of the Alps:

‘I must confess,’ he observed, viewing himself from head to foot, ‘that it costs a vast deal of toil and trouble to attract notice and to make a noise in the world.—Between ourselves, we should be much more comfortable around a blazing fire and in the Capital.—My two colleagues ¹ are enjoying themselves at my expence [sic]. Cambaceres, with his feet under the table and close to an excellent fire, is indulging himself to excess, while I am actually freezing with cold. But, patience, my turn will soon come.’

¹ The Second Consul was Jean-Jacques-Régis de Cambacérès (1753-1824); the Third Consul was Charles-François Lebrun (1739-1824).

2-: From Alderman Wood, published in John Bull (London, England) of Sunday 29^th July 1821:

There never was, perhaps, an instance of conduct so inexplicable as that of Alderman Wood on the day of the Coronation ².
[…]
[…] We must say a more extraordinary piece of conduct never has fallen under our eye. The Queen’s ³ own behaviour is certainly of the most particular cast—claiming to be crowned, and then asking to have a box to see the Coronation; and then, being refused that, falling back upon her former request, and desiring again to be crowned AS last Monday. But then the Queen is a droll eccentric body, and loves hoaxes and fun, but that the grave addle-pated Matthew Wood should be led, either from a love of turtle, or from a curiosity not natural to such mind as his, to tuck his feet under the table at a Royal Banquet, and swell the numbers of the train in the King’s triumph, is beyond us to understand.

² This refers to the coronation, on Thursday 19^th July 1821, of George Augustus Frederick (1762-1830) as King of the United Kingdom of Great Britain and Ireland under the name of George IV.
³ Caroline of Brunswick (1768-1821) was the estranged wife of George Augustus Frederick.

3-: From a letter to the Editor, dated Norwich, Saturday 30^th March 1822, by a person signing themself ‘W. S.’, published in The Times (London, England) of Monday 1^st April 1822:

Sir,—Old Cobbett ⁴, the bone-grubber, paid a visit to this city yesterday; and a pretty reception did this notorious impostor meet with! For some time past, he had caused it to be announced in our papers, that he would on the 29^th instant “dine with the farmers of Norfolk, and his friends of Norwich.” But, after all the exertions of the noisy fellow who acted the part of buffoon to the mountebank on this occasion, to the honour of this large and populous city be it told, that though 33 persons (most of them labourers from the plough tail) were present, only nine of its inhabitants could be found to put their feet under the same table with the wretch who promulgated the plan “for the forgery and sowing of bank-notes;” and even of those nine, not one could be induced to take the chair upon this occasion. Never was impostor treated with greater contempt!

⁴ William Cobbett (1763-1835) was an English author and political reformer.

4-: From The Times (London, England) of Monday 23^rd October 1826:

A right hon. gentleman ⁵, whose visit to Paris may not have been without its effect, both in deciding the French Ministry to issue the declaration respecting the new States of America, inserted in the Moniteur, which we noticed in The Times of Saturday, and in leading to a final resolution on the Turkish question, is said to have received at the hands of the King of France an honour which no Minister, foreign or domestic (with the exception of Prince Metternich and the Duke of Wellington), ever before received. Mr. Canning was invited to dine with his Majesty. It has been no uncommon thing for the Kings of England, for the Emperors of Russia, and many other European Sovereigns, to dine with their subjects, or with foreigners of distinction below the rank of Princes; but the Bourbons, both of France and Spain, would reckon it a dereliction of their royal state to place their feet under the same table with any but royal or princely personages.

⁵ This refers to the British statesman George Canning (1770-1827), who was then the Foreign Secretary.

5-: From a letter to the Editor, published in The Bath Chronicle (Bath, Somerset, England) of Thursday 23^rd November 1826:

Sir,—Since the enactment of Turnpike-gates on our highways, that daring species of robbers called highwaymen have become extinct. We no longer hear of any Turpins or Boulters; but perhaps for that very reason the by-ways have engendered a more sneaking class of plunderers, men who are better English geographers than any who have ever been retained by either our Courts of Justice, or the General Post-Office. It will doubtless appear strange to the majority of your readers, as I must own it did, only within the past week, to myself, to be now for the first time informed of the fact, that horse-stealers, or the transporters of any other plunder that may be conveyed on horseback, have only to pass one gate out of the immense deposit of tangible property presented by Bristol and Clifton, (between which there is no turnpike) to reach as far as the city Gloucester without any such interruption. Happening to mention this newly-acquired knowledge (which ought to give power over so much evil) to a respectable innkeeper, he confirmed it by the following relation. About four years since, a friend, having lost two horses, was induced to apply to a man residing in this neighbourhood, who was supposed to possess much knowingness of the marts for that description of free traffic; and the knowing one acknowledged the claim for his friendly assistance by stipulating that each should be provided with a horse qualified to make a long journey. Accordingly they set out through the very gate to which I have alluded, and actually travelled, without passing through another so far as Christchurch in Hampshire, where the stolen horses were found, and restored to the owner without further difficulty to either party. Total prevention of crime, whenever it can be accomplished, is better than the exercise of partial means of cure, however cleverly applied. I too seldom find my feet under the same table with magistrates or justices of the peace; nor will my ordinary avocations permit me to be the first to do every body’s business for payment by nobody, in dancing a mysterious attendance; but if any of your readers who are qualified recipients of what remains to be communicated on this subject feel disposed to avail themselves of it, a note from yourself would command the requisite confidence in their discretion, without exaction of fee or reward for my pains.
Clifton, Nov. 20, 1826.           B. D.

6-: From the review of The Club-book (London: Cochrane and Pickersgill, 1831), published in The Examiner (London, England) of Sunday 4^th September 1831:

The Club-book is neither a book on Clubs, nor for Clubs. It is simply a collection of tales in three volumes, “by various eminent hands.” It is put together on a principle long-established in the art of puffery: viz, that a good name is better sale than a good thing, and that, à fortiori, a constellation of good names must necessarily ensure a market. The Club-book is not, consequently, the result of an association of kindred spirit kindling at each other’s genius, and the bond of union is the publisher: the inspiration bank-notes: in short, The Club-book is a true bookseller’s knot. The writers who have here clubbed their pens together, probably never clubbed their feet under the same table in their lives.

7-: From The Welshman, or General Advertising Chronicle for the Principality or Wales (Carmarthen, Carmarthenshire, Wales) of Friday 25^th January 1833:

A SMUGGLER’S TALE.
[A STORY NO LESS STRANGE THAN TRUE]

Upon the eastern coast of Suffolk stands, upon a high cliff, the village of Pakefield, notorious alike for its spirituous people and spiritual pastor. It was formerly the haunt of many a bold and adventurous smuggler, and even in recent times, an occasional freak of that description takes place, and the Church itself has been the depositary of “a run.” Who is there that, by chance, some time or other, has not found himself, with his feet under the table of one of the hospitable farmers in the surrounding neighbourhood, doing ample justice to their “tea dinners” of strong ale, beef, ham, eggs, toasted cheese, and tea—and, to crown the whole, a “dash of Pakefleld cream” in the last cup, as a settler for that part of the ceremony, or as a prologue to the pipes and tobacco that are sure to follow?

8-: From Draughts of Character (London: William Carpenter, 1836), by ‘A. Corkscrew’—as quoted in The London Literary Gazette; and Journal of Belles Lettres, Arts, Sciences, &c. (London, England) of Saturday 20^th February 1836:

“The alderman’s favourite maxim it the exact reverse of the ancient axiom, ‘a contented mind is a continual feast.’ With him, a continual feast is the only source of a contented mind. His experience furnishes a contradiction to another venerable dogma, that ‘one swallow makes not a summer;’ for his summer is but one swallow. In his philosophy, good living constitutes a good life. He thinks that the way to ‘pursue’ happiness is to tuck your feet under the table. He cannot understand why so many thousands annually starve; if they are hungry, why don’t they dine?—if thirsty, why do they abstain from the bottle? King Solomon, he observes, ought to have married that French princess who inquired, when the people complained of the want of bread, why they didn’t eat buns!”