For this, probably the easiest is to set up a common directory/a few common directories, set up proper permissions through use of groups and worst case create some symlinks from the user's home directories, if these directories really need to be accessible from within their home directories. That's pretty much how shared directories are always done. As this would be a one time effort, it would be doable. But at this point, you should really think about paying some company with deep knowledge of Linux that can come up with a sustainable plan. Because obviously, your way of doing things isn't anything that could (or on that note should) be a long-term solution. And maybe think about rewriting the ancient software that causes this setup in the first place. Desperately trying to cling to something that has been out of support for decades is just not sustainable, not on any OS.
Richard