MU / pro site probleem whit users, mainsite/subsites

Is there any way to make this:

have the user that only can enter content on the mainsite (like forums ect) / admins can register there subsites.

user from the main site cannot login the subsite, there for they need to register at the subsite.
I tried membership plugin and some other wpmu plugins to get this to work but still not working.

I tried simple-press to, as forum in the subsite.
but simple press redirect registrants to the wp-signup.php so the users end up registering at the mainsite olso the subsite wont see the registrants in the subsite and they will end up in mainsite.

what's the best solution to get this to work?

1. how can users register at the subsite so admin will see this user in his dashboard.
2. close out users from the mainsite, so they cannot enter the subsites. unless they register at subsite. subsite and content must stay public. except the forum where the user need to register for.

was working on this for 10 hours today but still dint find anything good yet. so if any one have the solution i will be very thankful.