Online help

The Invite button works when the person you wish to chat with shows with the green online check.  With that name highlighted, Click the Invite button. The Chat invitation panel is displayed.

Type your message, include a subject if you wish.  The other people who have already joined this conference are shown on the invitation panel.   Click OK.

This opens an invitation panel invitation panel on your recipient's computer:

This panel gives the person you have invited a chance to decline.  They can give a reason for declining if they choose.  The other people who have already joined this conference are shown on the chat request panel.