We are a small installation of 4 users.  When one of us opens a top-level assembly the others are being locked out of editing all of the subassemblies and parts under the main assembly.  We have PDM but so far have elected not to use it.  Shouldn't I be able to open and edit the parts and subassemblies even if someone has the main assembly open?  Right now what we frequently have to do is to ask the one using the main assembly to close it while the rest of us open the parts and subassemblies we want to work on.  Are we missing something?  It doesn't seem like we should have to do this.