To launch a rocket with a neighbor, you need to meet the following requirements:
1. You should be neighbors in the game.
2. Both of you should have a Launch Pad and Mission Control Center of the same kind.
3. Both of you should have the four completed Collections for the launch of this rocket.
4. One of you should start a launch, and the other one should choose the friend in the launch start window. To do that, one of you should charge the collection, press Start, choose the neighbor in the appearing window by tapping on their avatar, and press Join.
Then you'll prepare for the launch together by gathering the necessary items.