I want to join the community; should I sign up for Slack or the groups.io mailing list?
Both subscriptions are sync'ed, meaning that if you subscribe to one, you will be invited to the other one; if you unsubscribe from one, you will be removed from the other one.
What do I use Slack for, and what do I use the groups.io mailing list for?
In general, you should use Slack for most discussion or questions for the community. The groups.io mailing list will send email to the whole group, so that should be reserved for important announcements. Of course, Energy DEN is still a work in progress, so for now it's fine to experiment while we all learn what works best for this community.
How do I change my subscription to get more/less email?
There are lots of options, all the way from nearly no mail to instant delivery whenever a new message is posted. Check out the subscription help page for details; note that you may need to create a password if you are a subscriber but have never logged in before.