Adding a 'post to everyone' option in Drupal Commons
Ok, I think I have enough information to help. Just because something is listed as a Group Post (i.e. something that can be posted into a group) doesn't mean it needs to be. In other words, by nature of not selecting any groups you are effectively leaving the piece of content out of the group context. 

Then, I might consider simply modifying the view which powers the user feed (this is a view that ships with Commons) and including non-group posts with a specific flag/tag. So to recap:


Create a tag (using taxonomy) or a flag (using this module) that effectively denotes a piece of content being non-group specific. e.g. public
Instruct your users by way of a bit of help text on the node type that they should use that tag and not post into a group.
Modify the view powering a user's feed (I think this is called Activity Stream IIRC) to include your newly tagged or flagged content.
Fancy bonus: Use a bit of JavaScript to disable the group field if either of the above flag/tag conditions are true. (Pseudo-Code: if node has tag public then disable list box)


That should get you there. Hope this helps.