Your userid is also munged in various complex ways (mainly by adding '.cam.ac.uk', or '.ac.uk' if it already ends 'cam', to the end of it) to produce the address your shared secret is emailed to.
Typical userids are rrw1000@cam, or empp1@cus, or aloan@ast. By default, WrenGROGGS will choose <userid>@cam as your userid, where >userid> is your identifier on the machine you're on. This may not work in some cases - eg. if you're in astronomy you'll get a default userid of something like aloan@cam, which is clearly incorrect.
Before you register, WrenGROGGS will prompt you for a 'correct' id. You should give your preferred email address, omitting 'cam.ac.uk' from the end if it is something like 'cus.cam.ac.uk' or 'hermes.cam.ac.uk', and leaving 'cam' in if it's just <user>@cam.ac.uk. Hence aloan@ast would enter (assuming he wants to process GROGGS-related mail at 'cam.ac.uk') ajl23@cam. If you're using WrenGROGGS, you should remember to type that userid back into grogprofile when you enter your shared secret, or you won't be able to reply to GROGGS.
In general, if the system you're running WrenGROGGS from is cus or thor, the default userid will set mail to go to wherever you've redirected '@cam' mail (ie. hermes, by default).