sysadmin@cwcmh.org, we aren't on the Webex Suite yet, but we have had issues in the past with changes having to be made in the platform it was scheduled in. For example, if the meeting was scheduled with the Webex Scheduler in MS Outlook, you have to make all updates for that meeting in MS Outlook. For the meeting you are trying to edit, did the user create it in MS Outlook? If it was created in Webex, they need to manage their co-hosts in the app or web portal.
FYI, I just created a meeting in MS Outlook with the Webex Scheduler and was able to save it and then go back and change the co-host although, as I said, we're still on the old platform.