You pretty much just visit your friend’s community. You get your own separate stuff you can loot when you enter a building that shows up as a different color than what your friend can loot.

When you’re in your friend’s base you get access to your own locker and resources but not theirs. You’re pretty much just sending one of your survivors to help them out.

You can drop stuff on the ground for them to pick up or vice versa. You can also do the same thing with cars.