This happens to me all the time at school. A group of people need to meet and we can’t figure out what time works for everyone. It would be great if everyone used their Outlook calendar but lots of people still don’t use an electronic calendar. So, we go back and forth via email and struggle to find a common meeting time.
Technology can rescue us from this situation. Several websites exist that allow people to agree on a meeting time. An easy and effective site is http://moreganize.com/. You create a new event (in my case, a meeting) and go through a three step process: event information, proposed dates and proposed times for each date. Then you just send the link to your group. Everyone can select the times they can attend and even add comments. You can easily see what date will work for everyone and no emailing back and forth!
Give it a try.