Splitscreen questions.
For 4 person Co-Op you have a few options. If you have 4 friends online playing you can Invite them into your Backyard Battlefield, or you can join theirs, and form a team of up to 4 players with the player whose Backyard you're in being designated Group Leader- so if they came to you, then you are Group Leader and if you when to someone else then they're Group Leader.
When you have visitors in your Backyard the single player quests, L.E.A.F. Agent, Top Zombie, Delivery Quests, get locked out until you are alone in your Backyard again.
The Group Leader can then carry the other members of the team into a Garden Op, Graveyard Op or into Infinity Time where you play with your team. If they enter Multiplayer you can't be guaranteed to stay together because if neither side in Multiplayer has room for all of the people in your team you'll be split up between Plants and Zombies.
If you don't have friends online you can join Garden Ops or Graveyard Ops and either start your own game session and wait for other players to join you, either before starting or joining in-progress, or you can elect to search for and join a Co-Op game which is already in progress.
