How to automatically select the organic group audience when adding content?
OG group extras


  Extra functions, blocks, and views for Organic Groups 7.2. In
  particular, this module adds views and blocks needed by sites that are
  not using Panels.


This only works on OG 7.2 but not on OG 7.1. If you are using OG 7.1, you can use panels and add a create group content panel to the node page.