C:\Users\[your user name]\AppData\Roaming\Dustin Leavins\Leavins Inventory Manager\
No, Leavins Inventory Manager does not support multiple users. This is because multi-user systems can be very complicated to use, and Leavins Inventory Manager was built primarily for ease of use.
As many as you need; Leavins Inventory Manager's only limit is the size of your hard drive.
There is a helpful tutorial on how to use a barcode scanner that should tell you everything that you need to know about barcode functionality.
Please Contact Dustin Leavins for answers to any additional questions that you may have.