It seems grouping means, if you put two bus stops facing each other on both sides of the streets, they get grouped together and appear as a single stop. Grouping/Ungrouping modifies that behaviour, maybe sometimes it doesn’t get grouped automatically?
It works for all station types too. Useful for if you make a bus/tram line in a town/city and want to connect it to a rail station, by grouping, the passengers/cargo can switch lines at the grouped stops. Basically just like an interchange.