BuddyPress.org

Where in WPMU’s backend is this “missing” user not showing up?

Is it in the “Users” submenu?

If so, does the user show up under the “Site Admin > Users” submenu?

If so, that means that the “missing” user has their own blog. The “Users” submenu only shows those users that are subscribed to your blog–in this case, the site’s main blog. The other menu option shows all registered users on the site.