I you want to coop with a friend after you started an island, make sure you put your stuff you want to keep on their island such as Grass, Diamonds, armor, food, and blocks or it will all get removed. ~just a little tip so you wont go what I had to go through~
I joined someone's island a long time ago when I was wealthy. I had lots of valuables on me. Then when I joined his island my eery thing in my inventory and my hotbar suddenly disappeared. That lead to me quitting the server. But now I'm back. But now I know why, thanks for that.
