Crew members do get tips you've added to your onboard account. You go to guest services, provide the name and position of the crew member, the amount of tip and you'll get a confirmation slip which in turn you give to the crew member (I usually write a thank you note to accompany this). The first time I put a tip on my account for my butler, he told me they get the money on payday which, for him, was at the end of the month.