Members cannot sign up for Buddypress or new blog

Hello,

I have installed WPMU and Buddypress on my main domain. New members and bloggers can sign up either as a member or for a new blog with no problems.

However, when a visitors tries to sign up through a member's blog, they get sent to http://subblog.mainblog.com/register and get this error message:

Permission Denied
Not Found / No Access

The page you are looking for either does not exist, or you do not have the permissions to access it.

Does anyone know how to fix this?

Thanks,

Ed