It's not possible to add custom fields to groups unless you create an integration to do just that like the Wall, Events, etc.. integrations do. GJ is not a profile, nor is it a field manager or anything of the sort.

GJ 2.3 comes with an events integration. That should be sufficient for schdules. As far as the rest you'll probably need to develop custom integrations to allow logging and displaying of the information you need/want.

Sorry, personally have no references for any other developers. Only other developers I know personally are other extension developers like me and the rest of the CB Team, which they don't do custom work. You may want to check joomlancers or other contract sites or attempt to take on the project your self after reviewing existing integrations. Creating an integration is relatively easy and is typically only 2 files. I recommend starting with the About integration as it's the simplest (just review).