From SA Mafia Wiki
Masons are players, typically townies, that may speak to each other out of thread. While Masons are usually confirmed townies, several games on SA have scum Masons within a Mason group; if there is a probability of a scum mason, it is expected that mods will include a line similar to "You are not sure of Player's alignment" in the role PM. A mod telling a player their mason buddy is town when in truth he's scum is seen as incredibly unethical.
The advantage that Masons share is having another (usually) trustworthy player to discuss the game with, as well as discuss opinions of other players. Masons can also protect their partners from being lynched, and Masons tend to be one of the more easily confirmed roles in the game.
Oftentimes a mason starts with a Mason Recruiter, a player who targets others at night to recruit into the Masonry. If the player is town, the player becomes a mason, and may discuss OOT with the other masons; if the player is scum, typically the recruiter dies.