What do you mean "I made sure that all users have access to the file". You do NOT want to give them access to the file directly. If you are using instant web publishing, they should be connecting via web browser by typing your computer's IP address.
Does your computer have any firewall turned on?
Sorry I didn't clarify. By saying that all other users had access to the file I meant they had read-only privileges. I do not have any firewall turned on and when the other people try to type the IP address in, the browser just spins and doesn't connect (they are also on the same network as me). Any help is greatly appreciated.
Have you read the PDF document on web publishing that comes with Filemaker? There's a number of settings to check--including whether a specific port is open and this is documented in that document. You can get to it from your Help menu in Filemaker.
They will not gain access by just typing in the IP address: they have to append the port number :591 to the end.
(Although I accept that you must also have done that to access it even on your own machine.)
Also, check the Privilege Set that the users have been assigned to - they have to have the Extended Privilege of Access via IWP ticked.
Try this: go to their machine and type http://<yourIPaddress>:591 into the address bar, and log on as yourself.