How to automatically add the permission "edit" on a node to a user who is not the author? I'm in D6

I have a node type "Person" and a node type "Company"

Each node "Person" has :


an unique field-node-reference to a node "Company"
an unique field-user-reference (to a user)


So, several persons can belong to the same company

Use case : 


The person P1.1 (belongs to company C1) creates a node (article)


How to automaticly grant edit permission on this article to an other person (P1.2) who belongs to the same company (C1) ?

Thank you
Install Content Access and ACL, and then use Rules to add Person 1.2 as a person able to edit that content.

You may find this question helpful: Node access by role or user, what module(s) should I use?

The Content Access module gives you role permissions at the content type and (optionally) node level. By default, node level permissions are disabled but you can enable it on a type-by-type basis.
If you install ACL module, Content Access can give you per user permissions at the content type and node level.