Question
Why can I not edit or add cohosts to my Webex meeting?
Answer
Check if you are the Host, you can only edit the meeting if you are the host.
Check if someone is in the meeting, the meeting can only be edited if no one is in the meeting. One indicator is the green dot signifying someone is in the meeting
If you are not able to edit the Webex Meeting through the Webex site or the application then it was probably set up through Google Calendar using the Webex extension.
Go to Google calendar and find the meeting. Select the meeting then the edit icon from the pop-up.
Further Reading
Edit or Add Cohost to Webex Meetings