I use Remind for work, and it works excellent. It's designed for school settings, but you use the same logic for any setting. What I like about it is that there are options to have only the creator ('teacher' in their language) to send, but then there are ways to keep conversations going two ways between creator/teacher and individuals, and entire group to each other. The latter has to actually be enabled and set up by the creator/teacher, so if you don't want it, just don't set it up. I didn't.