Adding a field without the field_ prefix (Drupal 7)
The Organic Groups module has a nifty API method for this. You just need to do a:

og_create_field(OG_AUDIENCE_FIELD, 'user', 'user');


This first checks if the field exists and creates it if it doesn't, then checks if an instance of the field has been attached to the user entity and creates it if it doesn't.

You can check this answer out to understand where and how you can run this snippet from.