OmniPlan doesn't support collaboration right now, this is something we're looking at for OmniPlan 2.0. Right now you have to designate one person as the project file "owner". If there are any changes an employee wants to make they should tell the owner to make the changes.
Not sure if you're interested in something like this, but a possible alternative is to set up a version control system. You can store the Plan file on a server and every employee has to check the file out, make their changes, then commit the modified file back to the server so that it becomes the latest revision of the file. You can read more about this type of workflow here:
http://en.wikipedia.org/wiki/Subversion_(software)