Hi @PAtlan,
Unfortunately, out of the box it’s not possible to make associations using workflows.
You have two options if you’d like to only do that within Hubspot :
- You have operations profesionnal and can write a custom code
- You can use a marketplace app called associ8 (check their docs, they do have working examples) that will add new actions in your workflows like associating a record with another from a separate object (custom or not)
In both cases, you would indeed make your workflow a site-based workflow, and in the workflow triggers select : contacts > create date is known.
This way any contact that is associated with the site will trigger it.
Here is an example with a test custom object on my portal (you’d replace bateau by site in your case) :
Then you would use one of the two ways aforementionned to make your association with the company.
Hope this helps ! If it does, please consider marking this as a solution 
Best,
Ludwig