I believe this link
explains everything you need. Especially check the table in the end.
It's actually pretty generous compared to what I remember from PnP (AD&D).
Which was straight up division. If you killed a monsters worth 100 XP:
1 character alone would get 100xp
2 characters would get 50xp each.
5 characters would get 20xp each.
Except that, in PnP, this only counts permanent characters (like PCs and henchmen), and not summons (who don't get XP (not that it matters), but don't count against the party size).
Also, in 3e (but not 3.5e), if you add a lower level character to the party, the rest of the party will actually get *more* XP due to XP awards being based on average party level, and not the individual character's party level. (Of note, in Icewind Dale 2, XP works as in 3e, allowing for this sort of exploit, and the rule against gaining multiple levels at once isn't implemented, allowing you to get tons of XP if you delay leveling and only level up some characters.)
Also, starting with 3e, the "A" ("Advanced") was removed from the acronym, so it's now just "D&D".