usability issue. when removing host from a group, that host has to be moved to the other box, which displays hosts from another group.
1. if user just wants to remove host from a group, that is confusing, because it seems to imply that host has to be added to some other group always;
2. if user would actually like to assign host to the other group, at first it looks like that would happen (host is listed in that list), but such an operation is not performed.
maybe redesign the screen so that hosts can only be added to the left listbox, and to remove them button 'remove' is introduced below the left listbox ?
might still be confusing, maybe there's a better solution.