User information is kept in the system.mdw file. Here is an example of reading data using the ADOX library to access the system information.
Function JetSecurity()
'-- set reference to ADOX library
'- Microsoft ADO Ext. 2.6 for DDL and Security
'-- Microsoft ActiveX data objects 2.6 library also needed for ADO
Dim cn As New ADODB.Connection
cn.Provider = "Microsoft.Jet.OLEDB.4.0"
cn.Properties("Data Source"

= _
"C:\Program Files\Microsoft Office\Office\Samples\Northwind.mdb"
cn.Properties("Jet OLEDB:System database"

= _
"C:\Program Files\Common Files\System\System.mdw"
cn.Open UserID:="Admin", Password:=""
''cn.Open
Dim cg As New ADOX.Catalog
''Set cg.ActiveConnection = CurrentProject.Connection
Set cg.ActiveConnection = cn
Dim ur As ADOX.User, gp As ADOX.Group, pp As ADOX.Property
For Each ur In cg.Users
Debug.Print "users = "; ur.Name
'- 0 = no permissions, the -number is a bitmap for a set of enumerations
'' Debug.Print "permissions = "; ur.GetPermissions("IDTable", adPermObjTable)
'' Debug.Print "property = "; cg.Users("jerry"

.Name
Next
For Each gp In cg.Groups
Debug.Print "groups = "; gp.Name
Next
End Function