- Can the user create the Group? is it any other way to do it ?
- is it possible to create the subgroup under group?
- Group administrators to have authority to Approve/reject group
Yes, you can create wiki posts.
Groups can be created only by admins (maybe moderators?), which solves the problem of admins approving groups.
There are no sub-groups. You would need to create the “sub-group” and manually add the members of the group to the sub-group.