A few years ago, I was teaching a graduate class and had to cancel a full day due to snow storms. I was looking for a way to have class virtually through a chat room. At the time, there weren't many free services out there. In fact, finding a service without paying an arm and a leg was virtually impossible.
I just recently came across a site called ChatFoo, which creates a temporary chat room for you to use. Simply go to the website and click on the "Start New Chat" icon, you will be able to create your own private room. This may be a helpful place to have study help available for your students. It could also be useful if you wanted to facilitate an online discussion with your students in a computer lab setting. It may also be useful if you wanted to chat with a guest speaker.
In order to get others to participate, they will need to have the link to the chat room. This appears in the top-right-hand corner of your screen under "Share this Room." You are also able to share this link through social media outlets like Facebook and Twitter.