File Sharing Permissions Problem When NEW Files Created

My wife & I have a home office network setup for our business, and are having trouble with shared files over the network. We're both on Macs, and all of our client files reside on an internal hard drive in my Mac Pro. I've set her up as a "Sharing Only" user on my computer (which matches the username & password of her account on her iMac), and have properly setup file sharing in both the System Preferences, and in the directory itself that we want shared. I've also applied the permissions to all files & folders below the main folder, and she can view, open and save whatever she needs and it works great.

*Here's where the problem comes in...* If either of us create a NEW file or folder, the other person doesn't have WRITE access to it (we can browse and open it fine). The second I go to update a file she created, it won't let me over-write it, and I have to go into the permissions for that specific file and give myself Read/Write access. I've experimented with "Groups", and assigned both of us a special group, and applied that group access to that directory, but it doesn't make any difference when a NEW file or folder is created - it doesn't copy the parent directory permissions to itself, it makes one of us the owner, and nobody else has access.

Is there any way around this? We're both constantly creating new files, and it really hampers our workflow to have to keep re-applying file permissions every time one of us creates something new. Ideally, any new files created within that main client folder would automatically have the same permissions as the parent directory. Any help would be greatly appreciated! Thanks!!