How to set blog post under event section in Drupal commons
Entity reference

Not sure if I correctly understand the question, but it sounds to me that using the Entity Reference module might be the solution you are looking for. Here's a quote from the module's project page:


  Provides a field type that can reference arbitrary entities


References

Another option is to consider the References module, which provides D7 versions of the 'node_reference' (and 'user_reference') field types, that were part of the CCK package in D6.

Recommended tutorials

If these module don't help for what you're trying to achieve, then reviewing the amazing set of videos to Learn Organic Groups might help somehow.