Hi @cristianorsini,
You should use the category mechanism for that, not tags.
Simply place entries for each user under a specific category. If you require a login for your site, you can use the entitlement mechanism to prevent users from seeing assets placed in categories they are not authorized to see.
https://knowledge.kaltura.com/introduction-kaltura-entitlement-infrastructure
and:
https://knowledge.kaltura.com/faq/how-set-entitlement-settings-kmc
This is how our own portal - MediaSpace [KMS] accomplishes this task.