See campus map http://www.lsce.ipsl.fr/Images/Page/1/Plan_Site_Gif.gif
It will be either at the castel (number 03) : main meeting room + coffee breaks or at LSCE (number 12, meeting room) : breakout group.
The campus is not far from the RER B station (name : Gif-sur-Yvette). The closest airport is Orly, and the closest TGV train station is Massy TGV. It is on the direct RERB line from CDG airport (but exactly at the opposite / Paris), so depending on the time in the day you need about 1h15 once in the RER to reach Gif-sur Yvette from CDG. You need about 1h from Paris.
* Restaurant open on Sunday or Monday evening in Gif-sur-Yvette or Orsay https://files.lsce.ipsl.fr/public.php?service=files&t=bea6a90b0c2aa37bcacf7d60d6d62486
* Meeting dîner : location and map https://files.lsce.ipsl.fr/public.php?service=files&t=9b3e8def4ad4927247dabfcebe034905
Rooms are available either at Hotel Le Village (near the main entrance of the campus) or at CNRS (on the campus). You SHOULD MAKE YOUR OWN BOOKING rapidly, we cannot guaranty that the rooms will remain available.
a) For Hotel Le Village (70/79€ per night) see: http://www.hotellevillage.fr/
Their site seems to be in French, but you can find it easily on booking.com or equivalent.
b) For the CNRS campus (~44€ per night, but I suppose the price is different for standard or prestige room), you need to fill the xls file https://files.lsce.ipsl.fr/public.php?service=files&t=ea5d95de4a0276f843c3f7e21b232a81and send it back by email and email only to :
accueil-gif@dr4.cnrs.fr
The file is all in French, but I included comments in the places you need to fill in. For the billing information it would certainly be easier that you put your name, pay directly by CB and then be reimbursed later by your institution.