I have set up workflow that will populatte a field Registration Number when a contact is associated with a company by an association label called ‘Primary Contact’.
The association between company and contact is irrelevant for the enrollment of your workflow. It does not affect enrollment at all: For enrollment, the workflow only checks whether there is any company record in the CRM where “Registration Number is known”.
If you want this workflow to trigger whenever a new contact is associated, you would have to add an additional property to the enrollment criteria. “Registration Number is known AND Number of associated contacts is known”. Re-enrollment should be enabled for both.
Now, when you associated a new contact, the Number of associated contacts updates automatically, HubSpot registers the change (via “is known”) and the company will be re-enrolled, triggering the copy action to be repeated.