The Party Escort Bot, also known as the party associate, is an Aperture Science robot partially cut from Portal, and reintroduced in the game for the retconned ending featured in the update made on March 3, 2010, as part of the Portal ARG.
The party associate is first mentioned by GLaDOS after Chell escapes certain death at the end of Test Chamber 19. There she says that they are throwing a party in honor of Chell's tremendous success, and that a party associate will arrive shortly to collect her for her party. GLaDOS then orders Chell to assume the party escort submission position or she will miss the party. As Chell disregards GLaDOS's words, the party associate is never seen at that point. At the very end of the game however, when Chell finds herself on the Aperture Laboratories parking lot after eventually destroying GLaDOS, the party associate finally comes to pick her up, and drags her away inside the facility, thanking her for assuming the party escort submission position, while remaining unseen.
As the end of Chell's series of tests did not proceed properly, the bot is forced to reach the outside to pick her up. It is likely that during normal tests, the bot merely waits for the Test Subject outside the last Test Chamber, and directs them to the party, if it ever was part of the tests. Given its voice, it is possibly of masculine programming, and may not be the sole Party Escort Bot to have been used. Given the sounds it makes, it also seems to be mounted on two legs.