OK, I can see that. I guess I have not described the situation properly.
I work with cakePHP and include access control in my app as described in 'Auth and ACL an end to end tutorial' (part 1 and part 2). I was trying to model this situation for the tables 'aros_acos', 'aros', 'acos', 'groups', and 'users' as a learning exercise in MR.
So, I can see now that I am not looking at a PK - FK relationship linkage defined in sql, the program code is handling the selection of the appropriate tables to assign access to controllers (ACOs) by users and groups (AROs).
I appologize for my confused request and thank you for your time.