I suppose you could have a related group table, not sure what to name it, then the added columns in users would identify what group and role a user has.
User 1 (create a users_roles table if you can have more than 1 role, or just a role_id under the users table)
- User Role (roles or groups table)
--- Add (roles_permissions table)
--- Update (roles_permissions table)
User 2:
- Reviewer Role (roles or groups table)
--- Review (roles_permissions table)
--- Approve (roles_permissions table)
User 3:
- Admin Role
--- Add (roles_permissions table)
--- Update (roles_permissions table)
--- Review (roles_permissions table)
--- Approve (roles_permissions table)