Current look for DefaultGuest
A guest is a special user for players wishing to not use a ROBLOX account. Guests are known as "The True Testers of ROBLOX" by the administrators. Either way, a guest is a user playing ROBLOX without an account. When a player hovers the mouse over a guest image when viewing game servers, the name will appear as "A Friendly Guest".
Many users create fake guest accounts (eg. Guest123, Guest_8164, etc.) and dress like guests. However, many people can identify if they are a guest or not, because users cannot leave spaces in their username. They can also be recognized since guest names do not have underscores, and all guests have one to four digits in their username (eg. Guest 1337).
Functionality
If a player joins a game but does not register or sign in, they are labeled as a guest by default. The guest's avatar is a placeholder for players interested in a game and want to play it before creating an account. The placeholders are BillyBloxxer, BettyBloxxer and DefaultGuest. When guests play games that save data (like money), they will lose all their data when they leave.
ROBLOX stores a cookie on the user's computer that keeps their ID. Cookie editors are able to change a guest's ID number by changing the UserID in Guest Data.
From 2008 to 2014, guests were able to speak by using the proper Super Safe Chat words. With the removal of Super Safe Chat, guests are unable to communicate, and they have to create an account to chat.
It is possible for scripts to ban all guests from a game. However, this violates the terms and conditions, because it makes some guests believe that ROBLOX is a bugged game.
Appearances
BillyBloxxer, BettyBloxxer and DefaultGuest are the ROBLOX guest templates. Joining in-game as a guest will load one of their character appearances unless the developer of the game uses a 
Script to change this.
DefaultGuest, BillyBloxxer and BettyBloxxer, before July 12, 2016.
When guests first appeared, BillyBloxxer and BettyBloxxer had very similar appearances to the user ROBLOX. Their only difference is that Billy Bloxxer wore the Red Roblox Cap, while BettyBloxxer wore the Pinktastic Hair. The DefaultGuest doesn't have an accessory; merely black limbs and a grey torso.
DefaultGuest, BillyBloxxer and BettyBloxxer, after September 13, 2016.
On July 12, 2016, the appearances of the guests were modified for the first time. The male guest (BillyBloxxer) now wears the ROBLOX Jacket, the Black Jeans, and the True Blue Hair. The female guest is similar to the male guest except with the Lavender Updo Hair, which looks similar to the Chestnut Bun, but a different colour. The other male guest (DefaultGuest) is still similar except that it wears the ROBLOX Jacket and The Black Jeans. On September 13, 2016, guests began to wear the Robloxian 2.0 package and the ROBLOX 'R' Baseball Cap. Sixteen days later, the R15 body was applied to guests. After ROBLOX introduced a new logo, the guest's shirt changed to the new ROBLOX Logo Jacket on January 16, 2017 (six days after the logo began usage).
Gender
With the addition of new updates, both new users and guests can pick a gender for their character. Users can choose the male guest, which wears the ROBLOX Jacket, the Black Jeans, and the True Blue Hair. Users can also choose to be the female guest, which is the same as the male guest except it wears the Lavender Updo Hair. If playing without choosing your gender or playing on mobile, users will be loaded as a genderless guest (DefaultGuest). All guests that join a game in the iOS/Android version of ROBLOX will wear a ROBLOX jacket and jeans, the 2.0 body, and the [1]ROBLOX 'R' Baseball Cap due to not being able to choose if you want to be a male or female guest in the iOS/Android version.
Critism
Since many guests are new to ROBLOX, may will carry out activities that users find annoying, such as deleting people's creations, trolling, or causing their team to lose. Guests cannot communicate with other players, though some games had guest talk scripts that allowed guests to communicate. There have been various petitions on ROBLOX to remove guest, but Telamon clearly stated that guests aren't going to be removed.