GuestData

The GuestData cookie is a browser cookie that assigns a player's in-game name if they are a guest. It will always be negative, no matter what. The last four digits are the common numbers guests have at the end of them, like "Guest 4182". These numbers are randomly generated every time the guest navigates the site, so it is possible to start off with 4182, click 'Discover' and then have 3486.

Editing
People were used to be able to change their guest number by editing the cookie using tools such as EditThisCookie. Some YouTubers taught players how to change their guest number to 666 to scare others. However, with the removal of guests in October 2017 and all methods of becoming a guest being removed in November 2018, it is no longer possible to use a legitimate and official method to play as a guest.

Alternative Method
Rarely known, but there was a code that could be found from the Roblox website when you select your gender through Guest Mode. You can find this code by Inspect Element. The code was only visible when you enter a game through Guest Mode.

roblox-player:1+launchmode:play+gameinfo:Guest:-(GUESTID)+launchtime:1519425624744+placelauncherurl:https://assetgame.roblox.com/game/PlaceLauncher.ashx?request=RequestGame&browserTrackerId=14353303313&placeId=(GAMEID)&isPartyLeader=false&genderId=(GENDERID)+browsertrackerid:14353303313

(GUESTID) = Negative ID of Guest. Leaving the (GENDERID) blank will give you access through all Guest IDs from 1 to 2147483647. That number specifically being the eighth Mersenne prime.

(GAMEID) = Game you want to join

(GENDERID) = The Gender of your Guest


 * 1 = Cap Guest (Gender Neutral)
 * 01 = Cap Guest (Gender Neutral) w/ access to pick (GUESTID)
 * 2 = Male Guest (Blue Hair)
 * 02 = Male Guest (Blue Hair) w/ access to pick (GUESTID)
 * 3 = Female Guest (Pink Hair)
 * 03 = Female Guest (Pink Hair) w/ access to pick (GUESTID)
 * nothing = Cap Guest (Gender Neutral) from Cookie Editor w/ access to pick (GUESTID)

When you enter your code into the searchbar and press enter, it would load you into the game with the genderid and guestid of your choice.

Fun Fact: Before the Guest Removal of 2018, if you added an infinity sign into the GuestID, you would always join the game as Guest 8.