I open-sourced a tool I created to scratch an itch. From the downloads for the installer on the project page I can see I'm not the only one interested. About 5 people seem to have upgraded from the previous version. But I know next to nothing about them.
Do I need a news group? A mailing list? Or how would you start to build a (little) community?
If you have webhosting, the simplest to set up is an online forum or bulletin board (e.g. phpBB, vBulletin.) I used to an open-source tool for Nagios and shamelessly used their mailing lists for support, and people started e-mailing me directly using my e-mail address from my posts.
I see you're hosting on Google - Google Groups does newsgroups, so maybe set one of those up for your tool?
It mainly depend what kind of interaction you want with them.
With such a few people the mailing list or a news group looks the most appropriate.
A forum with such a small number of people is generally not very active. (From other people it may looks like an inactive project).