You can book a hotel room before/after the conference from the following web-sites.
List of hotels near Narita airport can be fond at Narita airport travel support .
If you want to find a better deal, check out Rakuten Travel.
It is also a good idea to stay along the JR line connecting Narita and Zushi (Hayama). The JR railway runs some hotels at the stations. Here are the list: Kamakura Ofuna, Kawasaki and Tsudanuma are on the line. Another good choice is Shinagawa Price Hotel, which half way to Hayama and connected to the Shinagawa station. You can find more hotels at Rakuten Travel.
If you need any help, please feel free to ask one of the organizers.