Hi Mark - thanks for the submission. I'll answer your questions in reverse order - easiest to hardest.
There is no limit to the number of members you can attach to your boat. Basically, a member for us is a person who is given the right to reserve your boat for a certain percentage of the month. A member set up with a 10% share would be able to directly reserve your boat 10% of the time on a monthly basis. When you add a member to your boat you are also adding them as a Nautical Monkey user, but we don't send them advertising emails or sell our emails to anyone. They would only get emails associated with them using the site, notification of reservations, confirmation of reservation requests, etc. The user also has the option to limit notification emails in their account area.
As far as adding crew members to a reservation that is an interesting idea. Those people wouldn't be "members" of your boat under our paradigm because, presumably, you wouldn't want them scheduling the boat directly - looks like you would just want them to be notified of an upcoming trip and be able to opt in as a crew member. If I got that wrong let me know. Let me give that some thought and see how we might be able to implement the concept - we have been thinking of doing some infrastructure for crewing for some time and this might be the opportunity to work through it. I'll let you know once we've had a chance to work through it and feel free to contact me with any other questions or additional information.