@Low Your app concern could be solved in multiple ways. One would be to be able to create a group in a way that maintained compatibility with other, existing apps such as email and mailing lists, but only provide enhanced experiences for DC users instead of locking others out (akin to progressive enhancement in web development).
If this would not be feasible with features built into DC itself, it could still be worked around with a hosted bridge/relay bot that interfaced with encrypted groups for you, not unlike what is described here for another purpose:
Another alternative would be a lightweight web app: