Registration at Networked Blogs

I am running wp multisite with buddy press right now. (although I may remove bp before launch)

Question is, how does a visitor to a site on the network register at that site? All the register links point to the main site on the network.

If someone wants to join the site to submit a classified ad, they are sent to sign up at instead as far as I know this will not make them a member of the site. so there is no point in running a classified ads site on any site on the network, as no-one can register at any of these sites.

How do people register at individual blogs on the network?