The official site for Luigi's Mansion: Dark Moon mentions the game will have local multiplayer, yet the game is still listed a single player title.
The official website for Luigi's Mansion: Dark Moon makes the following statement:
"With so many new places to explore, all new ways to capture ghosts, and a brand new local multiplayer mode, Luigi's Mansion: Dark Moon is another star turn for Mario's heroic brother-and it's only on Nintendo 3DS."
The product details for the game, however, still read "No. of players: 1."
Nintendo has not officially announced whether Luigi's Mansion: Dark Moon will have mulitplayer, and to what extent it would play out in the game.
We have contacted Nintendo for confirmation, and will update with more news as it is presented to us.