Does a dungeon Guild Achievement still work if members of the party are offline?

In WoW ofcourse. What if a party member disconnects during the boss fight for example? I'd like to do quicky achievements for my guild, but not many people are around so filling party with alts will just have them show up as offline unless they were kicked. Will they register for the avhievement? You need 4 guildies in a party to get it.