Set User to be able to see a selection of customers but not all

If we have a level 1 staff member we might not want them to have access to say, the AD at a huge clients site or similarly a certain A grade client. I can see that we can give them access to either ALL or ONE client but not SOME. Any thoughts here? Would significantly enhance the product allowing for more users that would only need access to a few (but not one) client. Thoughts?

Hey no response here, Bump?