Nicole,
When you have Express Invoice on one computer, it is your host. You turn on web access from that computer for other ones to access it. You do not install Express Invoice on the other computers. If you do, it will not sync with your host one.
In your PREFERENCES, go to WEB ACCESS. This will provide check boxes for local and internet access. Basically, anyone on your local network (in your office or home) uses the local. They would put that Local Network address provided into their browser's address bar and then log in with the user name and password you have provided them. Anyone not in your local office or home would use the Internet address in their browser's address bar.
You can provide user names and passwords in the WEB ACCOUNTS portion of the PREFERENCES. When you click ADD, it will pop up a box that gives you a place to give them a user name and password, as well as determine what kind of access they can have in Express Invoice.
Hope this helps!