What version are you using?
From your main computer, where the program is installed, go into your preferences. From there, go into Web Accounts. You can set user names and passwords for people accessing it from your network on the web.
If you want it to be password protected on your main computer, I don't know of any way to do that from the program. You would have to have a login for the computer itself for just those people. But you really shouldn't keep your invoicing software on a computer that can be accessed by more people than just your staff, right?
Teri