how to allow multiple users to log in

I have a vb.net application with a database in MS access stored in a machine.The MS access file is stored in a machine and i have around 5 client machines which are supposed to access the MS Access file at the same time. the problem is only one user can log in at one time. whenever a second user tries to log in from a second PC i get an exception. what should i do?? help... thanks...