Hi,

It would be a interessting idea if it could be possible to have a leader associated to a group.

There is the same model for project in JIRA, you can have different users able to modify the members of a project.
However, groups are still editable by the administrator only.

It would be nice to have a administration screen accessible to group leader where a user (or a list of user) could associate or disassociate people from groups.