Add Ability for users to be aligned with multiple Companies

It would be helpful to create Companies to house each of our client's Projects to improve the ability to silo client work, however currently a user can only be assigned to a single Company, which causes issues with this approach when users work on several clients' work. I want to be able to set a user as belonging to (and having access to) our "Primary Agency" Company, Client A's Company, and Client B's Company, but not Client C's Company.