Hi Steve,
mojoPortal does create those users when new sites are created under related sites mode, but if you login in as one of those it should not really be an administrator unless you change it back to not related sites mode. If you find those users are really admins in related sites mode let me know.
The rationale for creating them is specifically to address the problem that if you create a site in related sites mode and then change the setting so that related sites mode is not enabled, you would have no admin user in the site. If it isn't related sites mode then child site admins are created as admin@admin.com when the site is created. However this would create duplicates under related sites mode and is not allowed so we append the site id, thus we get admin2@admin.com and so on.
Best,
Joe