need some advice about building a type of membership site

I just wanted to pass this by you guys, maybe you have some ideas about whats doable and whats not.

I've got a client who needs to offer registrations for two types of users, lets call them brokers and clients.
Both will get different user rights and roles, I am pretty sure I can handle it so far.

None of the registrations shall be paid but both require admin permission - I think manual moderation will be fine but having a registration page that offers links for both would be great. I mean a link that says click here to become a broker and click here to become a client (or a drop down).

Based on the role/user rights they will see different content and be able to access different parts of the website.

Now comes the tricky part:

I would need the brokers to be able to add new users and each broker should only be able to see his users.

I have currently no idea how to do the last part I listed as the tricky part. I thought about enabling multi-site and redirecting all blogs to the main-site as separate sites are not needed nor desired but that would solve that part...

Any hints or advice for any part of this mission are more than welcome.