Hi @antonionuzzo 
There isn’t a way in Databricks to restrict workspace admins from inviting users who aren’t part of their workspace but already exist in the metastore. If you're trying to track this activity, you can use the system.access.audit table (assuming system tables are enabled).
Here's a sample query you can run to check for user additions in a specific workspace:
SELECT
     event_time,
     user_identity.email,
     action_name,
     request_params
FROM
     system.access.audit
WHERE
     action_name = 'addUser'
     AND request_params.workspace_id = 'YOUR_WORKSPACE_ID'
ORDER BY
     event_time DESC;
This should help you to see when users were added and by whom.