Webminar/Chat style plugin

Hi

One of my clients wants to set up a webinar/presentation style thing via the website where people’s comments will be uploaded automatically rather than having to refresh your page etc. The only thing I know and can suggest is Webex but that comes with a cost and they only want a chat session every few weeks, perhaps only 3-4 times so there is no point in taking on a cost. Can anybody recommend a plugin for WordPress that we could easily install to allow this? A normal chat option isn’t sufficient as there will be a group of people who will join the chat/presentation at one time.

Many thanks
Kathryn