limct,
Permissions are usually handled on an operating system level. People can have read/write/execute rights to files. Moreover, these are divided up among owner, group, and other. So it is possible prevent people from not writing to your dwg file, but read it only. You can add them to a group to allow writing to a certain project dwg. This is useful when working with projects or groups of engineers.
To change the permissions (in windows) right click and change it appropriately. In unix use chmod ugo+rw-x.
Hopes this helps.
mfelzien