You can place a refundable deposit to have your name put on the waitlist. As the dates for the trip are determined, it is likely that some spots will open up.
IMPORTANT NOTE: Travel dates have not been determined, and won't be until the dates of the Rome temple open house are announced. The first half of 2018 date range is therefore approximate. For updates on construction status, see the official Rome temple page at http://www.ldschurchtemples.com/rome/
For the itinerary, other tour details, and to reserve spots, see Natural Italy's tour page: