If a group has all of these qualities, then it will be deleted.
- Unowned
- Manual Approval
- No members
ex: http://www.roblox.com/groups/group.aspx?gid=622912
After the group is deleted, the name will be usable again.
But if nobody is in the group and it is manual approval then nobody can accept you into the group, and therefore it should be shut down to make room for more