This is not a direct answer. I just talk a little louder. The beauty of an ebike is that you can all ride close together. The slower person can keep up and the faster person does not have to slow down on the hill and overheat or exert more perceived energy slowing down. If I want to, I just slow down or speed up to talk to my buddy and then, if I want, I can speed up again or slow down again. That is my idea of a group ride on ebikes - where people want to hang out together and ride as a close group or a close subgroup.
The other choice is to use a phone - you can call using voice, pick up with voice, change volume with voice. I change the names of my friend to single letters so the phone don't get confused. Instead of say "call Anthony" and "call Antoine" I say "Call Mr. T", "Call Mr. J". Also, it helps if everyone has a way to charge their cell phones on long rides.