Groups is a way to display more questions on the same page on the mobile device.
Groups can be added like any other question, just drag the group question into your form:
Once dropped, type the group header (required) and click "Edit Group" to enter the group editing mode:
In group edit mode, just drag any input as usual. You CANNOT have a group within a group. You can have branches though.
To exit the group edit mode, click the left arrow on the top left, to go back to the usual hierarchy form editing: