I need normal users to mandatory define a group they belong to when they submit a given content, while allowing admins to leave the target group empty (that is, that content will not be associated to any group).

I tried to use hook_form_alter() with $form['og_group_ref']['#required'] = TRUE; but it didn't work (while it works for other "normal" fields).
I have also tried to change myModule weight, but that also didn't work, so I am left to.. make a og support request ;-)

Is there anything that og does to the og_group_ref field that inhibit form_alter() to override its #required property?