Hi @jozanini,
Please see answers to your questions:
1. What happens when a meeting has external participants?
There are public and private embedded apps. Public apps available for use by everyone. Private apps are just for your organization users.
2. Can you have spaces with external participants?
Yes, you can have spaces with external participants. But you can restrict messaging with external users. Please see below URL for more details:
3. Are user IDs global across the whole platform ?
Yes, the User IDs are unique across Webex Platform. The User ID is based on email ID and you can not use same email ID to create different Webex account.
4. Can a user belong to more than one organization ? If yes, what happens with their ID ?
No, user with same email ID does not belong to more than one organization.
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Thanks, Vaijanath S.