Buddypress on members blogs only

Hi we want to turn buddypress off on a new WPMU site we are building BUT we want to be able to allow an auto install of Buddypress when every new member signs up for the own *domain blog. How is this done?

In effect our main site will not have the social features of Buddypress but our members sites will, hope that makes sense.

We have seen this done on a few sites such as http://solopracticeuniversity.com/ who are using MU together with Buddypress, any help would be welcome.