The only time that you would have problems with folks not being able to share the database is if you didn't set up Workgroup Security correctly.
What you need to do is to secure the database by creating accounts for each approved user. For each of these users, the only thing you need to assure is that they don't have Open Exclusive rights on the database. That is the ONLY possible reason that a database on a shared directory would not be shared properly - Open Exclusive would prevent any but the first user from being able to get to the database. And by default, users who don't join a properly set-up workgroup come in as Admin, who DOES have the ability to Open Exclusive.
Once you fix that problem, you should be good to go. Search this forum for topics Workgroup Security and Workgroup Setup and the like. You will find tons of articles. Heck, between me and Pat Hartman, that's probably 50 articles right there.