I've been to many many hotels where you must be on their list for the guard to let you by the gate. One that comes to mind is the Andaz Papagayo, Costa Rica. We wanted to see the Four Seasons that is on the same peninsula. We had staff at the Andaz call and make arrangements for us to be allowed to visit the Four Seasons. Once on property, we were told where we could visit and how long we could stay. We were at the Aman Sveti Stefan a few weeks ago. I believe you must be a guest to enter the island. There is a guard at the end of the causeway who checks on the list to see if you are a guest, then radios to staff on the island.