You can edit group attributes by selecting Edit Group Attributes from a group action menu in the user group list or by selecting Edit Attributes from the Group Actions menu available while editing a group.

Select a value from the Users' Role menu. You can assign a role of Admin or Contributor. There is no Reader option since any user who does not belong to a group with an Admin or Contributor role assigned will have their role set to Reader. If you select No Assignment it means that no changes will be made to the roles of users who belong to the group.