One of the hardest things about scheduling a one-off meeting, thesis defense, or coordinating volunteers for an event is reaching a consensus.
A typical method of dealing with the problem is via email, where everyone sends available/non-available slots to a person coordinating the event. The time required to converge to a "solution" is governed by the slowest responder.
If this time is too long (unfortunately all too frequent), it is possible that some of the originally available timeslots have been booked for something else, and have hence been rendered unavailable.
The process often needs iteration, and finding consensus can be a nerve-racking problem.
One of my colleagues, Peter Beerli, recently alerted me (the department really) to a nifty (but really simple) tool called doodle.
Give it a spin.
It is really easy to set up, and people can mark available timeslots for everyone to see, which is a big help. One can also add comments like "I can do this time, but that time is preferable."
It also integrates nicely with other calendar programs, if required.