They help the troubled and ensure that Mara (currently known as Audrey) enters the Barn every 27 years. The group grew and continued their mission. When Lucy appeared, they made sure she entered the barn.
According to Jordan McKee, In the "Escape to Haven" (#EscapeToHaven) webseries, that the Maze tattoo was originally used during medieval times as a way of branding the Trouble and ensuring easy identification of the supernatural. Later the The Guard would adopt the former mark of shame as their logo. However Dwight Hendrickson, later claims, in the same webseries no less, that the Guard are nothing but lies, so how much of that version of the Tattoo's origin story is true is left open to doubt.
The organization came to the attention of Nathan and Audrey around season 3 during the murders of the Bolt Gun Killer. So Nathan decides to infiltrate the group via Jordan McKee. But they where using him to get to Audrey. Using Ginger Danvers to control Nathan they got Lucy in the Barn, when she ran to stop the troubles for 27 years. In season 4 the Guard tried to kill Nathan because he destroyed the Barn and the troubles stayed and caused havoc to Haven. However Vince saved their lives and convinced the Guard to keep Nathan alive so he could find Audrey to kill him.
The nurse, who reveals to be a member, talks to Dwight and she questions him on Vince's whereabouts. In Speak No Evil, Dwight takes over the Guard from Vince after he fails to get protection for Henry Barrow. With control of the Guard, Dwight demands that Nathan hands Mara over to them. When Nathan fails to after he realized Audrey is still in there, Dwight orders the Guard to go after him.
Rhett is the only member who is not Troubled nor does he have the tattoo.