The full set of usertags are publicly visible via the debbugs data available by rsync.

You can query by usertag user with something like:

http://bugs.debian.org/cgi-bin/pkgreport.cgi?usertag=debian-qa@lists.debian.org

or add ;tag=value to limit by both user and tag.

Some users in use:

debian-qa@lists.debian.org

debian-release@lists.debian.org

debian-security@lists.debian.org