Your crew can only work on islands you have already completed. To add crew to start mining gold, scroll to the left to one of the completed islands, and click on the green plus sign. You can now use your in-game cash to hire crewmembers. Click BUY for whichever you can afford.
If you have more questions, please contact us through the game by going to our Menu > Settings > Support. We can help you quicker this way, because the game will allow us to view your account and better help you.