I'm struggling with configuring access rules for an entity (including sub entity).

I've an entity which holds data belonging to different departments. The access (also view access) should only be possible for users belonging to the department assigned to that data. I tried to use grouped users.
So I configured a field of type "grouped users" in that entity using a global list. Then I accessed users to the items in the global list. I set the access rules to "view assigned only". It does not work. I only get items shown, which are added be that user (field "added by").
So, what do I wrong?
Does any one has some instructions or an example how to get it done? Many thanks.
