To create a meting group, click in the three dot menu next to the attendees of the meeting. You'll see it in that menu.
Once you add a Meeting Group, you can add agenda items directly into the group by clicking into the three dot menu of the group itself.
Any Agenda Items that are NOT in a group will be at the TOP of the agenda. Because any agenda item at the end is technically part of the last group.
You can move Agenda Groups around and all agenda items will follow.
If you ungroup an Agenda Item Group, the items will still remain (they won't be deleted).