BuddyPress.org

I’m not sure we’re not talking at cross purposes here.

you say “new user was for main site” ok all fine and good but “mot a new created blog site” ? not sure what that means.

A new user registers they opt to create a new blog, they become a member of the main site i.e the parent WP site which is running BP they also get a blog created at a sub domain address this blog is in effect a standalone blog it should be linked to however from their profile. Iniatially they will be the only person registered to that sub domain blog and the main site has to allow users to register to the sub blogs.