BuddyPress.org

Hi, im running a BP + WPMU install. When an user signs up to the site their only added to that specific site as an user. Is there a way to have the user added to all sites on the network at once? or at least provide the option on the registration page? My main site is http://www.justfortheculture.com and a sub site would be http://www.sneakbid.com