Adventure Quests are designed for an individual player. Family or friends can easily tag along, but only the registered player will be able to interact with the quest. If you and a friend would like to compete, you will each have to register because only registered players/participants will be able to interact with the quest and receive the clues.
Discover Quests can be enjoyed by an individual, a couple, or by a car load of friends, but only the person holding the phone will be able to directly interact with the quest.