Seems like I didn't find any complete resource for doing this anywhere.
I already have an accordion group set up http://screencast.com/t/Bpubn8GvvI in a Profile type "account profile" (Profile2 module). I intend this profile type to be displayed at registration page. The thing is I need to code my user registration page using a TPL because the client want to rearrange the fields like it's a "fill-in-the-blanks" questionnaire.
I looked at the $form using dsm() and find it hard to print render() the group. All I see is a #fieldgroups, #groups and #group_children object which cannot be rendered.