New Users added to newest blog as main site- not main site

Hi, great plugin, but not sure how to fix this. My main site is mainsite.com. I have many subfolder sites… mainsite.com, mainsite.com/sub1, mainsite.com/sub2, mainsite.com/sub3, etc.

When a new user registers, they are assigned their main blog as the newest one (blog) that has been created (mainsite.com/sub3), NOT the main blog (mainsite.com).

So, I end up with some users having main blog initially set as i.e. mainsite.com/sub3 or 4,5, 6, 18, 32, 48 or whatever blog was last created before they registered.

I know that the user can re-set their main blog, but how can I have the plugin default their main blog to be the main blog in the system, i.e
mainsite.com

Thank you for any assistance!