Google Hangouts is the best free way to host a video conferencing session with multiple people. These are some resources that will specifically help you with joining a session, if you've been sent an invite.
Just in case you need a refresher, this is some more information about what Google Hangouts is and how it works.Edit Remove Move
This is really good. If you want an in-depth explanation of Google Plus, this is probably all you need!Edit Remove Move
If you're logged into Google Plus, it will be pretty simple to join a Hangout, as Hangouts are part of the Google Plus social network. This video is an explanation of how to do just that!Edit Remove Move
You can start or join a Google Hangout session through functionality built within Gmail. You can also accept an invitation to join a hangout in your inbox.Edit Remove Move
If you have an Android phone, it will work pretty similarly. You can access hangouts using the camera on your phone.Edit Remove Move
This is a very nice explanation of how to use Google Hangouts once you've already accepted an invitation to join a session.Edit Remove Move
Screen sharing is really easy with Google Hangouts. This means you can share a live version of your computer screen with the other participants in the Hangout. In fact, you can even hand over controls to them, if you really want.Edit Remove Move