Please make a poll/vote feature! This type of tool begs for it, and I was under the assumption it was included in the way it was described.
For example, it would work like this. Create a poll that says "Would dates X thru Y work for you?" Then, everyone votes and a percentage is calculated (i.e. "75% of invitees have replied 'yes' ")
(Note that Planapple already has a way that your tripmates can vote thumbs up/down on individual items in your trip notebook. This is a request for something a little different: a “survey question” for your tripmates, with multiple choice responses.)
Paul Quelet commented
Allow users to post a poll to discuss trip ideas. (This is very similar to the Travefy travel app that allows polls.)