Guest users can reset their password by visiting the following URL:
https://wiki.illinois.edu/wiki/forgotuserpassword.action
In order to complete a password reset, the guest user must enter their email address exactly as it appears on the space permissions page. The Wiki service will then send a temporary password to the guest user's email account.
(From here: https://wiki.illinois.edu/wiki/display/HELP/Managing+Guest+Users+for+Your+Space)