We did something similar for VRWorkout where group sessions were available as an external ical calendar. The problem was that one can not add an external calend

We did something similar for VRWorkout where group sessions were available as an external ical calendar. The problem was that one can not add an external calendar to google calendar from the mobile device. Only from the web calendar, which is kind of annoying. Not sure if they are using an external calendar, but what would be the best way to update a big group of people (that opt into it) about certain events that happen every few days ?