Hey there!
It's pretty simple to be honest. You just have to send him invite to your island with "/is invite (Your friend's nickname)". Then, he will receive a notification in chat about your invite. All he has to do then is just type "/is accept (Your name)". That's pretty much it. Now you two are on same island and he has all permissions.
Oh, and there is one more option, if you just want him to have permissions on your island but you don't want to fully share it with him. You just need to type "/is coop (His name)". I am pretty sure it will give him permissions to break blocks etc... but don't trust me on that one, I am not one hundred percent sure since I am not really a SkyBlock player.