Guest is the default anonymous user name assigned to any user who does not have a username on CollabVM.
When you enter CollabVM for the first time, you will be assigned the name guest(string of numbers). The random string of numbers can be anywhere from 1000 to 99999. You can also be assigned a guest name if your username is in use, have an invalid username, don't have cookies enabled or cleared your cookies.
You can choose to set a user name or keep the guest name; it's up to you.
The guest RNG is defined in the collab-vm-server source code; CollabVM.cpp, line 264. You can increase or lower this amount if you want.
The guest name itself is defined in CollabVM.cpp between lines 1553 and 1555. You can change this to something else like "anon" if you want.