Have you considered something like Eventbrite? This is an online event booking solution which includes payment and social media integration. We use this for managing some Agile South Coast meetings we host regularly in our Bournemouth office.
Thank you Peter. We did look at Evenbrite but there is an issue with the payment facility : we're based in Switzerland and Evenbrite does not offer credit card payment in CHF. People could only pay via Paypal, which might still represent a hurdle for some users here. But I agree - it's a great solution.