Guests are players who are in ShellShock without logging into an account.
They have all the basic functions of members, with some exceptions. They can gain Experience, however it does not contribute to leveling as guests are locked to the weapons and maps of the first level and cannot increase the level. As they cannot level, they cannot access weapons that tanks of any level higher than one can. They are restricted from using the chat. Stats do not carry between games and when the game is finally closed after a session.
They are easily noticed by the grey G that replaces level in lobbies and the fact that they will always have a "Guest" before their username. Unless otherwise specified upon logging in, the username will be "Guest Player (number between 1 and 999)".
Finally, they can be filtered out by members upon game creation.