Can other players visit my island and grief it?

No. Your island is fully protected from outside griefing. By default, only players you invite as island members can build, break, or interact with blocks on your island. Visitors can only look around and explore if you make your island public - they cannot damage or steal anything. Always be cautious when inviting new members, since staff cannot refund items if someone you personally added causes issues.

Tip: You can check the permissions of visitors and such in the /is permissions menu. Find it in /is menu.