Limit User Addition to User's Role or Below

I’m adding a user role for my company to use to manage plugins, etc for our client sites, and trying to limit the role of the client administrators. I want to keep them from adding plugins, themes, etc.

I know how to add a role and adjust capabilities via the theme (php). But what I’m looking to do is add a new Role called “[MyCompanyName] Admin” above Administrator. Then, I’ll remove the ability to add plugins, etc from Administrator. So here’s the long and short of it: I want the Administrator role to be able to add users, but not to add a Role above itself (aka “[MyCompanyName] Admin”).

Can someone give me some pointers on how I could accomplish this? Any advice is greatly appreciated.