The awesome Alys has made a HabitRPG Guild for Beeminder users: https://habitrpg.com/#/options/groups/guilds/dca65ca5-2bae-442d-a564-d1a1fb6a00b4
(If that link doesn’t work, just search for “Beeminder” in the public guild list.)
We don’t have any challenges yet (hopefully coming soon if we can think of some!), but we’ve got a nice conversation going about our goals and the best ways to use HabitRPG and Beeminder together for maximum effectiveness. So if you’re using both and you’d like some company, please join us!