BuddyPress.org

You are using outdated versions of both WPMU and BuddyPress. I urge you to upgrade.

There was a bug in some previous version of BuddyPress to do with the wrong link being put on to the activation emails, but without knowing exactly why people can’t activate their accounts (i.e. do they get the emails? Does the link work? What happens?) it is tough to provide specific advice.