I tried to search on dictionary but seems the term "party" is used in videogames "out of context".
Party: a social gathering, as of invited guests at a private home, for conversation, refreshments, entertainment, etc.
It seems to have nothing to do with a RPG Party (maybe in a loose interpretation). Seems that someone at some point just started to use that word and people liked the word for that use. Recluting someone (
Hei join the party!) into your group to go killing a boss is not exactly the same as a social gathering at your home.. or maybe yes? at least you are going to boss' home (uninvited).
Was that a carryover from tabletop games (see comment here)?