A guest can't sign in. They have to be a registered user. Registered users show up as guests when they don't sign in. Any time you look at the message board without signing in, you show up as one of the number of guests. You can't post or access some of the areas of the board until you sign in, then your username shows up at the bottom as a user.
When I approve a new member, I ask them to go to the General Discussion section and introduce themselves to everyone and tell us something about their car or the car they'd like to have. Some of them never respond, nor ever sign in again. I occasionally go through the registered members, and those that have registered and never logged in again for a period of one year, I delete their account.