This is not currently supported, but I am wondering why don't you assign your users, belonging to a group, to a specific role.
Actually, the membership provider could be overridden and extended. For example, you could implement the following logic:
1. Interpret some of the users as roles
2. Create an additional profile field, in which you will store the relations - several users linked to one (group holder)
3. Find the group users using the relations.
I will follow up if I figure out some other suggestion.
the Telerik team