A question about adding people to islands in skyblock

My friend and I have been trying to play together and we ran into a problem. Every time he does the /island add command, it just takes him to his island and when he does it at his island, it says I'm offline or not in the same server. if anyone could help it would be greatly appreciated. Thank You.

 

Tpa to him with /tpa [Player]
When u did that and he still can't add u, try to add him.
If that doesn't work maybe try /debug
if that doesn't work either I have no idea xd