Numerous backend engineers at Metamarkets and other companies work on Druid full-time. If you any questions about usage or code, feel free to contact any of us.
The best place for questions is through our mailing list: druid-development@googlegroups.com https://groups.google.com/d/forum/druid-development
Several of us also hang out in the channel #druid-dev on irc.freenode.net.