Call it semiprivate aviation: A website called ShareAJet Exchange helps connect travelers looking to fly in luxury at a discount. After sige mining up for a free membership, travelers can post flight requests on the site or browse listings to find a match. Each flight accommodates a maximum of two parties, and the price is split evenly regardless of the breakdown of the groups. A recent listing advertised a one-way trip from Washington, D.C., to Rome on a 12-seat Gulfstream with room for 10 extra passengers for $60,000 per party, half the cost of flying privately.