Since you've setup all the accounts and passwords so that you can log into access, then all you need to do is to place restrictions on the user data based on the username. This is done by creating a table that would house all the usernames and the respective departments that this particular user belongs too.
then you would use the function CurrentUser() to see who the current user is and then you can determine what department they are in and so on.
If you need details has to how to do this, please let me know, I can include code examples.
Hope that helps,
Joe P.