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Avoid being ambitious when writing out the itinerary. Remember, a group tends to move slower as you add more people to it. Begin sketching a basic outline for the group, leaving gaps of time between traveling sessions, activities, and between rest periods.  An easy way to create and share an itinerary is through Google Docs. Add the group members to the document and give them permission to edit it. A few phone applications are useful for creating and sharing itineraries. Try Travefy or Tripit. The first aspect to add to the itinerary is your arrival and departure. If you need to get tickets to travel, such as when flying, do it once the group agrees on the travel date. Use these dates as a beginning and end, filling in the middle part of the trip with activities.  Travel arrangements are often cheaper to make in advance. If you need to get plane tickets, for instance, get the tickets right away to ensure you lock in the price and number of seats your group needs. If you travel through a company, ask them about group discounts. Next, figure out where the group will stay. Large hotels are always an option, but look into alternatives. Utilize smaller hotels, resorts, villas, and rented homes to keep the group together. Make these arrangements in advance to reserve whatever accommodations the group needs. You can rent homes and condos through services such as Airbnb. Figure out how you will get to the hotel or wherever you decided on staying. You may need to come up with special arrangements, such as taking a taxi from the airport to the hotel. Note how much time it takes to get from place to place. Rent a car, buy bus tickets, or make any other arrangements you need to do in advance. Next, begin outlining the activities your group plans on undertaking. This includes sightseeing, tours, and shopping. Leave some buffer time between each activity. Book any activities that need to be arranged in advance.  Doing 1 big activity a day is a fair way to compromise. Being on the go all the time is difficult, especially when some group members want to relax. Avoid making the schedule too strict. A schedule that is too packed or rigid falls apart when something goes wrong. Call restaurants in advance so they can make arrangements for your group. It is difficult for a restaurant to accommodate a group of 8 people coming through the doors during the dinner rush. Lock in your reservations for the sake of enjoyment and security. You can use applications like Table8 and OpenTable to reserve spots at some restaurants.
Outline your trip itinerary on paper. Arrange your arrival and departure first. Book your living accommodations. Schedule any transportation you need. Write group activities into the itinerary. Make your restaurant reservations.