BuddyPress.org

You, as a site-admin, have the overall admin access over all users and complete site options. If you go to your Options in the Site-Admin menu, you can allow registrations to users and blogs would be the simple way. If you create the users, then turn on just blog registrations. Users will be able to create blog and have full control over “only” the blogs they create, not the entire site.

Trent