If you manage groups or email lists with frequently changing membership or lots of transient topics, come give flowcontrol.io a look (http://try.flowcontrol.io/).
We built an instant discussion group product that lets you turn any email thread into a discussion group by simply by cc'ing an email address. No signup, setup or installation required. It's not for everyone, but if you're looking for a lightweight solution, check it out - (https://vimeo.com/81385188).
We have a unlimited free trial period for early adopters. I'm happy to take questions too, just message me directly.