How to grant edit permissions to a large group of members?

Is there way I can grant, for example, the entire Relief Society access to edit a particular calendar? Of course, I'd like to find a solution that doesn't involve me putting individual names in a list.

No.

Nor do I think it would be a good idea. Calendar editors can schedule an event in any building of the stake - even if it's a misuse of the calendar it's placed on (like missionary meals).

That's on top of the entire RS being able to edit an event that someone else placed on there. In the case of the missionary meals, that means that Sister A could delete Sister B's appointment so she can have the missionaries on her preferred day. With that many editors, you'd have zero accountability.