If you're a business, you need to consider hiring a part time sysadmin or a consultant. What you don't know you don't know about networking and computer security can cost you a lot of money.
Not to mention, of course, that relying on the support of free forums to keep your business going is a rather questionable business strategy. Anything that's vital to your operations should be in-house
Chances are, you need to disable homegroups on your "server" and enable password protected sharing.