Word of mouth publicity is probably the most profitable exposure your membership site will get.
Word of mouth recommendation is more valuable to you than any affiliate program for one very simple reason. If a marketer says that your site is great, that is seen as marketing. If a member says that your site is great, that is seen as fact. Word of mouth is significantly more powerful than the strategic marketing you pay for.
Word of mouth marketing means more than one person speaking with another to discuss or promote your website. Email, forward to a friend, forums and chat rooms, and blogging are fantastic word of mouth means of promoting your site as are testimonials and reviews.