Please check the following:
This issue only occurs when the browser in use has stored information for an existing GoToMeeting account in the cookies and offline data. When the website tries to activate the meeting ID, it checks the ID against the credentials for the existing account instead of just connecting to the session. This causes an error to be produced when the credentials and meeting ID do not match.
What I would suggest having them do Evi is clearing their cookies on the web-browser they are using or even using another browser all together to connect.