First, recognize the difference between an advisory committee and a board of directors. A board of directors is a legal entity, with legal responsibilities and liabilities. You have to be very careful about who you choose for board members because they control your company. If you have investors, especially venture capitalists, they'll expect to be on the board. An advisory committee, on the other hand, is informal.
Advisors have no legal authority and assume no liability.
You can form the advisory committee to meet your needs and their schedules. You can just have a few people who've agreed to let you turn to them for advice. But I've found that forming an official entity forms a closer bond between you and your advisors. If you've asked the right people, they'll consider the request carefully before accepting, and when they do, they'll take their role seriously and be even more committed to your success. First of all, people you respect.