If by that you mean "type in multiple names at once" it would make much more sense.
Yep, actually I thought just of a second button (remove) below the the textarea for adding users.
Each member must have one default group. To "remove" a default group from a member you have to make another group the default for him.
Yep, I know. But if you remove a user from a group which is his default group phpBB must do anything to ensure that (I guess it switches "registered user" to the default group).
The same thing could happen if you had an option "remove from default group".
I know. A couple of weeks ago I had to remove about 50 Users from a group (long story). And it's even MORE efford to do that via the "manage user" pannel.
Imo this options belong also in the "Groups Management".