BuddyPress.org

Since you have taken a WP install and turned it into a WPMU install, there would be no BP data to import from your original install. Thus, you will have no BP data when install and activate BP–other than your Admin account.

Did you search the WPMU forums for instructions?

https://mu.wordpress.org/search/upgrading+WP+to+WPMU?forums=1

If so, as DJPaul says, each member will have to log in before their initial BP data set is created.