How to use field_groups in admin only? I want to use field_groups in my content type to make it easier to deal with a long content type, but I don't want that group (probably a fieldset) to be rendered at all on the client-facing side. 

How do I control that?
Use CSS to change the div created by the field group.  Set margin, padding, border and font-size to 0.

Don't hide the div because that will hide the contents of the div.