Select a single group and select 'Add'. This allows you to add multiple 'users' to a single group
Within group menu a button for 'Bulk Group Management'. This allows the selection of multiple groups. Multiple users are then able to be added to these groups. A checkbox, for example.
The usage would be for administrators of OnDemand instances who occasionally create users on bulk and then need to add them to multiple groups.
An additional use case is when a profile is being deactivated and need to be removed from all groups.