Basically what you are doing is suggesting a gathering like a Yahtzee tournament with your friends, a game of football, or a cookout. Before you would have to call or email each person to see when they could do it, adjust the time based on peoples schedules, call everyone back, worry if enough people will attend, etc. With Eventative you create the proposed event and send invitations in just a couple minutes.
When you set up your proposed event you state the date, time, location, and enter the email addresses of the people you would like to come and if they can see the the guest list. Next you determine the tipping point (the minimum number of people needed) for the event to take place, and when they should respond by. The interface starts with just the three initial fields at top left and smoothly morphs and expands one section at a time as the required information is entered.
Immediately the people you invited get an email telling them of the suggested gathering. As the creator you also get an email. You can see below, there is only one response available to your possible guests - "I'm In". By clicking it they are saying if the event reaches critical mass then they will be there. There is no need to do anything if they are not able to partake. If the guest list was made to be visible you would see a list of the email addresses invited under the "create your own" button. When enough people are in an email goes out solidifying the event. Likewise, if there are not enough by the deadline a notice will dispatch stating it is not on.
On the organizer's email there is a link to see the event status which takes you to your event control page. From here you can add more guests, send a reminder, or cancel the event. In addition you are able to specifically see who accepted the invitation and if they even viewed the email. The invitation email uses an image for the countdown that changes accordingly as time passes, and the server that the image is on knows if it has been displayed yet. This way to track web based emails has been around for a while and is safe.
I have already used it and absolutely love it's simplicity. You can sign up for beta test access from the homepage and follow them on Facebook. It was created by Bret Kugelmass. The demo video below does a good job of describing how it works and you can read the FAQ for more information.