You can find the ID of a user under Settings > Properties > Search properties > type 'Contact owner' and click it > click 'Field type' in left siddebar. You'll now see each user with their user ID in the column 'Internal name'.
Should you be referring to contacts in your CRM, not users, then simply look up the value for 'Record ID' for a contact. This is also the second number in the URL when you open a contact record.
You can find the ID of a user under Settings > Properties > Search properties > type 'Contact owner' and click it > click 'Field type' in left siddebar. You'll now see each user with their user ID in the column 'Internal name'.
Should you be referring to contacts in your CRM, not users, then simply look up the value for 'Record ID' for a contact. This is also the second number in the URL when you open a contact record.