Check in at GetYourGuide office, located near Café Madeleine, across the street from the train station Versailles Château Rive Gauche (RER line C). Do not go directly at the Palace of Versailles.
You can cancel up to 24 hours in advance of the experience for a full refund.
It's not hard to get there on your own and meet up with the group.
Also, Tuesday is a day when there are fountain shows in the gardens, and many people come to see those.