BuddyPress.org

That email is generated in wpmu-functions.php part of the wpmu core. It’s called during the signup process and that function is wpmu_welcome_user_notification(). The function that is used for blog signup notification is right above that called wpmu_welcome_notification(). The blog signup email can be modified in the wpmu backend, not the user signup email. It’s just plain harded coded.

My hope rose when I saw at the top of the fn a filter. The filter is just for the username, pass and some other info that is irrelevant to the body of the email. I don’t see any way to change it except by modifying that fn itself.