It has to do with the reference path. First even between the same types of computers, you would need a network path. Otherwise if user1 inserted from a local file path, that local file path would not exist for user2.
Sorry do not quite understand I think I may not have explained the problem in enough detail. If in the Mac environment I have a record that has a container field I simply add file (picture) to field by right clicking. I choose the link option and the file/image is inserted. That appears in all the Mac environment users. If I look at the same entry on a PC the field is not populated and it says it cannot find the file. I assume this is because in the Mac environment the path is different to the PC because of the nature of server set up which is a Microsoft based system. Hence if I wish to open the folder in the Mac environment it is done via smb://and the TCP/Ip address whereas in the Windows environment the folder is mapped to a drive. So the path is quite different in the two OS. Does that make sense?
That was the point I was trying to make. The platform difference in understanding the file reference is what is creating your issue. You have to have it so that it is translated for each type of environment. Also it needs to reside on a shared resource instead of the local drive.
See if this thread helps.