It has become customary on mafiascum.net to use /in to sign up for a game in a Signup Thread; the syntax can be confusing to new visitors, as it seems to add an unnecessary "complication" to the process, and they may wonder if their signup will not count if they omit the forward slash.
The syntax comes from IRC (Internet Relay Chat), an old form of Instant Messaging that dates back to the late 1980's. Commands in that chat program (and still in some modern IM clients) had to be prefaced with a forward slash to prevent them being sent as plain text to the other users. When a Mafia bot was created for use in the AIM (AOL Instant Messenger) based GL Central, this syntax was used to "signup" for the game. Eventually, the habit found its way into the Queues on both the Grey Labyrinth and mafiascum.net, and is now the standard.
This format is purely optional. Your signup will count if you just say "in" or something similar.