You can use headlines for structuring the registration form (see image 1 as an example).
Please go to
Overview => Settings => Manage Participant Groups
First, enable the expert settings in the lower navigation bar by clicking on "Expert Settings Disabled" or the cogwheel. This gives you additional options for editing the participant groups.
You can use the field "Prefix Text" to add headlines above the different participant groups (see image 2). This could give more structure to the presentation of the different participant groups.
To make the prefix text stand out, you can use <h3> to show it in bigger font and a separate line, for example <h3>Participants from Low GDP Countries</h3>.