Can not enter a computor on the network

We hav a small office network. All computer heve file sharing. The one computer can access all other computers. None of the other however can access this computer. The error message is something in the line of – access denied no permissions contact administrator. I have tried everything I know.What else is can I do so that the other computers have acces to this one. We would like to use it as a “server” for a spesific program. Ps I can remote access to this computer via dyndns.