Changing All Documents Permissions

[Klaatu]

I'm using Yosemite (MAC Mini). No matter what document I work in (Word, Libre Office, Text Edit), if I create or change a document and save it, the Permissions (privileges) change (if they already were set by my partner) so that only I have "read-write" permission, my partner's name doesn't even appear in the Privilege section, and the "Others" shows "read only".

We save all documents to my partner's MAC Mini, and I've changed the privileges on the folder (and sub-folder) where all our documents are saved. But is she sends me a document and I edit, the privileges change so that ONLY I have "read-write" permission.

What do I change on my MAC that will change all the document privileges so that my partner AND I have "read-write" permissions?

Thanx for helping.