First, I must say that here at Forget Computers, we really dislike QuickBooks for Mac and recommend it be avoided at all costs. It's a poor application with poor customer support from Intuit. QuickBooks Online is our preferred solution (and even QuickBooks for Windows is preferred over the Mac version).
If you still feel the need to run QuickBooks for Mac and you want to use the multiple user capability, you must know that this feature requires the QuickBooks Server app in addition to multiple licenses of QuickBooks. This app enables multiple users and handles the local backup of the QuickBooks database. (It's very important to backup QuickBooks locally, and then backup the backup). Contrary to the name, the QuickBooks Server app is not required to run on a server, and can be run from ANY Mac that has QuickBooks installed and access to the QuickBooks data file. You'll need access to this app often because when this app is running users can not change preferences or even run reports! <lame>