Guests are non registered ROBLOX users. They can play games and chat in Super Safe Chat mode. The purpose of guests is to give people who have not registered, a chance to try ROBLOX before spending the time to sign up. A guest's name will always be in this format : "Guest #". Ex. "Guest 1978". Any ROBLOX user with no space between "Guest" and the number, is not a real guest, and should be reported, as guest impersonation is against the rules. All Guests will take on the appearence of ROBLOX in-game and will have a negative userId.
Some long term roblox users get angry and irritated with guests because they are still learning, or because they don't understand the game and may accidently team kill etc. Do your best to be friendly and tolerant with guests, as they are potentially future roblox users! Treat them like a person, not an idiot. Do not publish or give out scripts that deny guests from joining games, that is against the rules. Guests are one of the main ways that ROBLOX gets new members.