There is now a form for setting up an account for a guest, at the bottom of this page:
which leads to
If the guest needs a Terrier card, then go to the Terrier Cards for Affiliates page here:
Lastly, for running meetings one usually needs to set up guest wireless:
this leads to here:
where you set up the account. You only need to set up one account for a given meeting. Some people complain about having to reboot to use the guest wireless, but as far as I can tell they do indeed need to reboot.