Add existing record to portal
I find a lot of info about creating portals and adding new records to a portal. I can't seem to find any documentation/tutorials about adding an existing record to a portal.
I've got a table companies and a table contacts. I've created a many-to-many relationship between both, so I can add new contacts in the portal located on layout companies.
How can I add an already existing contact to a company? Or add one contact to two different companies?
Thank you for any tips.