Signup threads (or sign-ups) are threads where hosts collect the players for their game. Each Mafia Forum can handle this differently, but usually this is a separate thread, created by the host after the design phase has been completed and an In-Game Mod signals the host that the Mafia queue has reached their game and they should proceed with signups.
Signup threads usually have the roles and rules of the game clearly outlined so that players may consult them and ask questions during signups. Signup is usually done using the same roster display used in voting, so that players can see who else has joined. Replacements / backups may signup in the same thread as a separate, secondary roster, as well as mentors.
In some games, where the game mechanics require it, signups can be done by PM to protect the identity of one or more of those playing, adding an extra layer of anonymity to the game (e.g. Puppet Mafia, Quantum Leap Mafia).