Look at this document. It will help you.
https://docs.microsoft.com/en-us/azure/active-directory/saas-apps/cisco-webex-tutorial
https://help.webex.com/en-us/g5ey83/Configure-Single-Sign-On-for-Cisco-Webex-Site
I think to enable SSO on WebEx meeting on site admin page (not via control hub) you have to reach out to Cisco to enable SSO. Once its done, you can configure site for SSO. You will be able to access site URL without SSO using below.
https://SITENAME.Webex.com/admin
Regards,
Alok
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I have done this in past multiple times and as far as you have email address matching for user, there won't be any impact.
Before you enable the full synchronization, run a dry sync to verify that its matching to currently users in the hub and not deleting any users. Also you can create policy for auto sync and apply necessary subscription during synchronization.
Regards,
Alok
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