In Skype, how can I hang up from a group call that I started without ending the call for everyone else?

The bad news is you can't since your PC is routing the call.

As some sort of workaround i installed a Skype on a little Homeserver of mine so i can call myself prior to the conference call and then invite the other participants, that way the server is the actual host and i can leave anytime while the server still maintains the conference.