VIMP has 4 pre-defined roles. These are Administrator, Moderator, (registered) User and Guest.
Any Administrator can assign one or more roles to any registered user in the backend (-> User -> Edit).
There are no actual role hierarchies in VIMP. You can only assign permissions and remove permissions. So theoretically you could even grant administrator permissions to all unregistered users. On the other hand you could remove the administrator's permission to edit its own profile.
So BE CAREFUL!