This is a super free tool and is quick to create an account. It lets you schedule meetings and events and you simply send a link for your event to those you want there, allowing them to add themselves and give their availability. You can also use it for add opinion polls.
Doodle can take the hassle out of agreeing a date
It says on their website http://www.doodle.com/that Doodle saves a lot of time and energy when you’re trying to bring a number of people together – and it does! We’ve been using Doodle a lot recently for work and personal events and it really does remove the headache and hassle – you can access events that you are attending and those you have organised from your account page.
Instead of using just one option, you can propose several dates and times and the participants can indicate their availability online. Immediately you can see what the best time is for the meeting and schedule accordingly. Its a quick and easy way to organise people!