I'm not real sure I want to know why you were deleting users in SQL directly here - if they didn't have SMTP addresses that normally points to a synch problem or an issue with the RUS service or something along those lines... regardless, you'll have to reconfigure the default DB.
If you're on 4.0(x) you can run ConfigMgr which is found in \commserver\ and under the advanced options down at the bottom you'll see an option to recreate the default objects - be CAREFUL - there's a couple options, the one you want is "Reset all default ocnfiguration settings while preserving all existing objects" - don't pick the other one that will blow away your entire DB... that'd be bad...
This should recreate all default objects including the example administrtor, the installer account, opening greeting call handler etc...
