The market exists, but it's in its infancy. The Snowden revelations blew a hole wide open in the privacy market, and that's why you're starting to see more and more privacy companies opening. I suspect it won't be long before one of them (whisper?) offers a cross platform, encrypted group chat like you speak of. But these things take a while to build.
if mobile/tablet-only is OK, try wickr (wickr.com).
It works fairly well for me. They have a $100.000 bounty for someone who manages to break their code/get communication contents and they're sponsored by the EFF.
The downsides are that it's closed-source and that there's no desktop client (yet).