I have Thunderbird installed on another PC. I checked and the invite attendees works correctly. The difference between the two PCs is that on the PC with the problem there are three email addresses vs one on the other PC. On the other PC when you invite an attendee the screen is populated with the only email address configured. With the problem PC it is not populated. It makes me think that there is no email associated with the calendar. Do I need to configure a default email address.

Alright I fixed it. Now can someone explain why what I did resolved the problem. I have three email addresses configured in Thunderbird. I right clicked on the "Home" calendar and chose "Properties". The email address in "E-mail" was not my main email. I changed it to my main email and everything worked. If that resolved the problem than why did it not show an attendee for the email configured? I did some more checking and determined that if I select the email address it showed initially it still worked, and it put that email address in the attendees. I configured "None" and the problem returned. I suspect that the calendar was not configured and when I selected it in "Properties" it populated the email with the first one available. Does any of this make sense?