I think you all are over thinking this thing. When I arrrive to sail on a friend's boat I just say that and the boat's name and they wave me through. If they are not leaving on your boat you don't need to do anything. I would however notify immegrations as you clear in that you will have additional crew arrriving or with your charter company if they handle it so they can add them to the cruise permit and parks trust certificate.

Travel is fatal to prejudice, bigotry, and narrow-mindedness, and many of our people need it sorely on these accounts. - Mark Twain