Guest is the default anonymous user name assigned to any user who does not have a username on CollabVM.
When you first enter CollabVM on a new browser (it can also happen for other reasons. You will also be assigned a guest name if your username is in use, you don't have cookies enabled, or cleared your cookies), you will be assigned the name guest(string of numbers). The random string of numbers can be anywhere from 1000 to 99999.
You can choose to adopt a 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 234. You can increase or lower this amount if you want.