Online help

Clicking the Invite button on the Chat screen displays the Chat invitation panel:

Explore help: click on picture!

Type your message, include a subject if you wish.  (If other people have already joined this conference, you can't change the subject, and the people are shown on the invitation panel.)

Click Invite.  This opens an 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.