There are ceratinly several ways to accomplish this but the best way would be to have a specific agreement type and document security template that adds specific users or user group automatically when the document is created in the system. These users should be given read/write access. If the access has to be granted / removed based on the status of the agreement, Scripting would need to be used to enforce non standard rules or give / remove access. For documents alredy in the system without the proper user access you could add the needed collaborators via a batch import / modification of the required documents.
Regards,
Gary