Two multi-select lists

I have two multi select lists for a user management screen that controls account permissions. The user can select any # of users and any # of organizations.

Multi-Selects: a.) users multi-select b.) orgs multi-select

The requirement came through to populate each dropdown with all their associated organizations whenever you select a new user.

What should I look out for? What if they select a ton of users and each user has many different organizations so I end up displaying permission controls for 25+ organizations that only one user has permissions on? Any good patterns/guidelines for this type of problem