The Masons are on the villager team. At the beginning of the game (on the first night), the moderator should have the Masons open their eyes and recognize one another. This means the role is essentially meaningless unless at least 2 Masons are included -- a Mason's sole "ability" is her knowledge of the identities of the other Masons. The Masons can not "do" anything special. In other words, they are a group of players who know each other to be innocent (not werewolves).
The recommended signal is a triangle made with the thumbs and index fingers of both hands. This is a rough approximation of the Masonic pyramid symbol found on the reverse of United States one-dollar bills.