We have done the following:Verify DomainClaim UsersSite linkingSSOCalendar featuresSSO is very helpful with authentication and provides our security team with necessary controls. Site linking has been great for administrative purposes. 

Did the problematic participants tried to rejoin the meeting, rejoin should have fixed the issue. 

HI , I have been using WebEx for a very long time ( more than 10 years ) with WebEx and this is the best platform ever to be used for meetings and chats.. Keep rocking. !!