Well, it turns out the problems we were having with multiple users (with individual log-on accounts in Mac OS) being able to edit graphics in HDe was due to a bug. I spent a couple days with M100 tech support trying multiple things that didn't work, including wiping the hard drive and reinstalling both the OS and M100 software. The next day the tech support guys tested the configuration I was using on their own systems. Later that same day I received an e-mail from M100 tech support with a fix, basically an Applescript that ran and repaired whatever needed to be tweaked. Now, everything is running great. Any user can edit graphics while logged in under their own account.
My thanks to the guys who helped me at Media100 tech support, Wick and Scott!