TiW - How does it work?

[Mom2six]

How does the Tables in Wonderland card work? We are considering annual passes, but are really debating because of the dining plan. If we get a TiW card, how exactly does that work? How many do we need to buy - does each person need a separate card? We go as a family of 9 (me, DH, 6 kids, and my sister). Does it really save any money after paying to buy it? As a large group, we are automatically charged 18% for tip. Is the 18% before or after the discount? (Not trying to get out of paying the tip - just curious on how it works. We usually tip on the non-discounted amount anyway.)