Each individual on Workplace has their own user ID.

You can find this ID by going to the individual's profile and copying the number from the url. For example the number, in bold, at the end is the user ID: https://mycompany.facebook.com/profile.php?id= 100022481289696
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You might need someone's ID when sending them a message through the bot:

To find out more about sending messages to another user here.