Discourse can certainly be used primarily via email, though I personally prefer the wider glimpse into topics available that working from the site allows.
The only two drawbacks to operating solely via email might be, in my opinion:
- the difficulty in distinguishing between topics available to everyone versus personal messages just from email notifications, and
- a lack of awareness when new topics of interest are initiated without having a user’s email notifications set to everything.
However, there might be solutions to both of these issues that I’m simply not aware of.
What other needs does your group have, @mdepasquale? Discourse has so many options and features available that you might find solutions to problems you did not even know you had. 
Off the top of my head as a user, Discourse could provide your group with functionality for
- mentoring
- unit- or module-based learning
- background communication
- content curation
- team-based project collaboration
- easily shareable content mechanisms
- resource building
- community building
- journaling
- small group discussions
- …etc…
Does any of that help? 
ETA: @mdepasquale you might be interested in these two pages, if you have not investigated them already: