Firstly the lead booker of the yacht should add you to the crew list using your email address. You will then be able to log in and view the booking to ensure it is legitimate.
If the crew have not yet paid for the yacht in full, you can make your share of the payment directly to us via your booking profile. If the yacht is already paid in full you will have to arrange payment internally with the rest of the crew. The amount you pay is something that you should discuss with the booker.