Workgroup problem

I work for the military and i am running an internal network with a shared hard drive on one computer. It is nessesary that all computers on this workgroup are able to access the HD. The work group consists of 13 computers and devises at the moment and i need to had one more computer to this workgroup. I only have one computer that i can do this with but, when i set the computer up on this workgroup then tried to access the workgroup the computer said it could not access do to permissions or unavailability. I know the computer is good because it can access the internet from that line just not the workgroup. please help.