I'm having some difficulties! Ok, I have installed

Hi I'm having some difficulties!
Ok, I have installed membership and prosite for my multisite network.
What I want to happen is, when a visitor joins and becomes a member on my site, which is set up as a buddypress community, I want them to get a free blog automatically after they become a member. Do I manually have to do this and set up each free blog? That seems so tedious! . I just want them to be able to register for a site and get automatic access to it. I've seen it done because I've had a multisite before and when they registered to become a user they were automatically given a site on the network.
How can I achieve this?
I've created a test blog through the network admin "sites" access and on the test site there's access to upgrade to pro. Which is perfect and what I want.
I'm just having trouble getting the initial free blog automatically set up for a new member..
I hope this makes sense!
Thank you for your time