Add Ability for users to be aligned with multiple Companies

[#1]

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.