How Can A User Register A new Subdomain Within My Network?

I want to allow users tp sign up for a subdomain on my network.

For Example if user name is john then at the time of registration he should be able to register john.colornymph.com

I have installed WPMU on my site and am able to do add this via admin but I want my users to do this themselves.

I have installed Pro Sites, Membership premium and buddypress.