given enough time, it wouldn't take long to build up a database, using PHP or Perl and a MySQL database, of the valid pages. Assume that you use something like the
HTTP-Lite Perl module, and that each GET takes ~1 sec to verify. Then assuming 8-character user IDs, which can only contain alphanumeric characters, you've got 36^8 = 2821109907456 possibilities, which boils down to ... 89,702 years of compute time. Whew! Man I probably should have computed this last part before starting this post ;-)
Yeah...you'd probably be better off simply trying to get shell access on the web server (this is fairly commonplace at my university) -- then you just cd to /home and do an ls -- instant list of users.