My b2evolution Version: 3.3.3

Hi,

I'm the main admin of the Blog system. A friend of mine wanted a blog for a group of people. So I opened his blog(a group one) and set him as admin with level 6.
I also created a group of member and fill it with some example member (level 1).
It works fine as the admin of this blog can create articles, news and so on, and give access to public or only to his group.

But the question is: how can I give him the right to add, delete and edit people from his group (and to no other group, if possible)? By rising his level? To 7? 8?
But how can I link the admin to his group?

Thanks a lot
Regards
Pierre-Louis