The absolutely best way of getting to the meeting is with RER B train from Paris to Orsay station. From Orsay station it's only a 10 mins walk to the meeting room.
From RER stations St Michel Notre-Dame, Luxembourg and Port Royal, all in the area where I recommend you to book your hotel, it takes around 30 mins to go to Orsay. All trains with destination St Rémy-Lès-Chevreuse stop in Orsay.
See timetable here: http://www.ratp.fr/
Train/metro map: http://www.ratp.fr/plan-interactif/
To go from the meeting to dinner, we take the RER B to Châtelet-Les Halles, and change to RER A (same platform) and go to Auber station. From there it's a 2 mins walk to the Opera.