locked Feature suggestion: Group calendar
I suggest this with some trepidation, as I know that calendars are just about the canonical example of the "how hard can it be?" fallacy. Closer to the "research team and five years" end of the spectrum than "a few hours".
That trepidation is reinforced by the botch job Yahoo did of the Groups calendar redesign in mid-2012:
Anyway, the essential requested feature is the ability to create reminders for events, and have those reminders post to the group. In my PTA groups I've used them for recurring meetings and special events. It is those pesky timezones and especially daylight savings time that are fraught with geographic (varying laws) and temporal (changing laws) edge cases. Even my relatively simple use case with PTA group members all in the same timezone gets mangled by the current Yahoo Groups calendar around every fall back and spring forward.
I'm not particularly interested in integration between the group's calendar and personal or other calenders (though others might be). That said, if there's a way for Groups.io to leverage Google calendar to achieve group reminders that might make the effort manageable, and theirs seems to work.