Traveler's Salat: When the reduction should be started?

Suppose I am on a journey to more than the minimum distance (three manzils or 77 km) to be considered as Musafir (Traveler) with a intention of staying less than 15 days. * When does the Qasr Salah comes to consideration? Immediately after the journey has begun, or whenever 77km is traveled? * Should I participate Jamat of Salat or prefer to perform salat alone?