Slack was suggested by Ben at the January 25 meeting. I've heard very good things about Slack, though it seems to be the sort of application that is rolled out at an organizational level as a substitute for internal email, so I'm not sure it is ideal for our use case scenario. There is a free tier but the feature set is quite limited, and beyond that it becomes expensive.
We do obviously have a discussion/forum facility right here, built into the DFM Wiki, which we have not really used up to this point but that I think would meet our needs. It offers nice threaded discussions that can be bookmarked by URL. It also has an email notification system, which you can enable for activity on topics where you have posted. You have to enable this in Special:Preferences; go to the tab "Notifications" and check the "Email" box next to "Structured Discussions".
