We strongly recommend that you book your hotel as soon as possible. Hotels in Amsterdam are very busy in the summer. Unfortunately, hotel rates reflect this...
You might want to consider renting an apartment, and possibly sharing with other participants. Some very nice areas of Amsterdam (e.g., the Jordaan, West, and parts of Zuid) do not have many hotels, but may have apartments available.
Below are some hotels you might consider, but you should certainly check the usual search engines like booking.com.
Click on the hotels on the map for (sometimes) more information. We make no claims about availability of the listed hotels! The green pins indicate quite cheap (but basic) hotels or hostels. The red pin is the conference venue. It is easily accessible by public transport (there is a tram and metro stop next to the university, and the Amsterdam Zuid train station is just a short walk away).