You will be able to gain Friend Points by grouping with other players. These Friend Points can then be used in the Theater to get new Scene Cards.
If you select a group quest as opposed to a solo quest you can get extra rewards by being the quest leader. If you are a follower in a group quest you will expend no stamina.
Fever builds up gradually while you are fighting and when activated will restore your health, boost your attacks, and halve the requirements for special attacks. Try to save Fever for when you are fighting a boss as this is when it will be most beneficial.
Tag teaming at vital moments of a battle is a good strategy as it allows another character with stronger HP to enter the fight and take over.