How to allow user/ blog registration?
When you have a newly installed WordPress MU you should allow new users/blog registrations first in order to allow your visitors to create their own blogs. This can be done via the admin area of your WordPress MU. Please navigate to Site Admin -> Options tab
Now your visitors can register and create their own blogs by directly accessing the required address https://new-name.yourdomain.com or register via the login form on your home page.
If you would like to start your BuddyPress Social Network in a few clicks, check the BuddyPress Hosting package.