We moved the Open Data Kit community from Google Groups to a Discourse forum at
https://forum.opendatakit.org and it's been awesome. Over the last year, we doubled in size (now up to 7,000 members and growing) and a lot of that growth has been enabled by Discourse.
We use the forum for exactly what you described, longer conversations that aren't necessarily tied to a closable issue. And we also use it for all our support requests. Handling support is great on Discourse because the new topic interface shows users (a lot of them non-technical) similar questions and it has great search and SEO.
I think for a small community, Google Groups would work, but my gut says Google Groups is not long for this world. Also, while mailing lists are workable for support, but I find them miserable (e.g., you can't move posts from one topic to another, you can't easily prevent duplication questions).
Anyway, I'd say try Discourse. And if you have any questions about started with Discourse or using it for open source projects, I'm glad to help! My handle is "yanokwa" on Gmail and HN and Twitter and GitHub and pretty much everywhere.
And I guess while I have HN's attention, I can give a quick blurb on Open Data Kit!
Open Data Kit (ODK) is a set of tools that replace paper forms. Our tools are mostly used by social good organizations to collect mission-critical data in difficult environments (e.g., collecting polio vaccination data on 1 million kids in Mogadishu [1], monitoring elections in emerging democracies [2], protecting forests in Indonesia [3]).
I love working on ODK because the project has a measurable positive impact on the world and we have a very kind and welcoming community. As one of the project founders, I'm a little biased, but if you are looking to get into open-source software, ODK is a great project to contribute to.
To get started, go to http://github.com/opendatakit, find a pinned repo, read the readme, and claim a "quick win" issue. Oh, and sign up on the forum http://forum.opendatakit.org and introduce yourself!
[1] http://www.emro.who.int/som/somalia-news/somalia-to-conduct-...
[2] https://www.youtube.com/watch?v=O-CGzo3m4PA
[3] https://www.youtube.com/watch?v=f7rJCgwV3-g