I actually would like to have control over all the notification content coming out of Wordpress and BuddyPress, but thought I'd start here.
I have looked over the WPMU 'HTML E-mails for WordPress' plugin and it looks great for branding email, but doesn't appear to give me control over what the email reads.
So currently it reads:
"... Your membership request for the group "testing_only" has been accepted. To view the group please login and visit: http://domain.com/challenge-sign-up/testing_only/ --------------------- To disable these notifications please log in and go to: http://domain.com/members-2/username/settings/notifications/ ..."
Thanks very much for your help.