Multisite interactions?

Hi,

I got a project in which:

The Main site:

Holds all the info.
All User Reg. is done on this site.
All user subscription data is stored in this site.

Now the client sites:

They have nothing on them, they just get info from Main site (Above) and display.
So a new user comes to this client site. User clicks on reg. link and they are taken to main site for registration, after reg. user is sent back to client site.

Now the tricky part, on client site, the user should see himself logged in, and should be able to view the subscribed content (Which is on main site).

How to achieve the tricky part ?

If you need any clarification then please let me know.

Thanks.