Creating a list as a calculated column, then comparing 2 lists

[mdex]

I'm not even sure something like this is possible.

Imagine have 3 tables with the following columns.

Requests
ReqID, Customer, Requestor

Requestors 
Requestor

RequestorGroup
Requestor, GroupID

I can use simple relationship and RLS between Requests and Requestors to that Requestors can only see their own Request records.

What we also have is Requestor Group. This is a list of Requestors which for one reason or another are allowed to view each others requests. If you have the same GroupID as another person, you can see each others. Am I able to use some sort of lookup/List to create a relationship I can use with RLS? The challenge lies with the fact that requestors can be in multiple groups.