Multiplayer act bosses (noob Q)

when joining somebody else's game, do you need to be in the room when the boss is killed to get credit?
these are games where the maker has not completed the last quest...

In some quests you have to be present. The ancients quest won't complete if you don't meet the level requirements.
For most quests, as long there is someone in the room who has not completed it then everyone in the party gets the completion.
Level 90 rusher kills Andy and everyone else is in town, nobody gets credit.
Level 90 rusher kills Andy and a Level 12 that already completed the quest is in the room and everybody else is in town, nobody gets credit.
Level anything that has not completed the quest is in the room with Andy when she dies (but not necessarily kills her), then everybody in the party in town also gets credit even if they never left town.

See my own question here for more information.

thnx!
now ill lurk the lobby and avoid

Duriel 's killing room