There is no way to set “preferences” for the calendar. We just have to remember to set the RSVP to “on”.
You might be able to do this by using the Feature Assistant Extension (https://discussions.scouting.org/t/installing-the-feature-assistant-extension/53647) and importing events. However, I haven’t used it since the calendar changeover, so (1) I’m not sure which capabilities made the jump and which didn’t, and (2) I’m not sure this was ever something that could be set in the “old” calendar.
