Online Travel Booking and Bidding Agencies - Booking a hotel for a group. Sites?

I´m looking at booking a hotel in Germany for 2 nights for 15 people. We should need about 10 rooms as some are willing to share. What sites do you suggest or do I have to start making phonecalls directly to hotels? Can I even get a discount? It´s a weekend in Frankfurt, and most hotels are wide open or even empty. I know that I should get either the Marriott or Mövenpick on Priceline for a reasonable price. Should I call them and ask for about the same price? I´d prefer the Marriott for this visit.

Look online and get an idea of the best rates for the dates you're looking at, select the hotel you want, and then call that hotel directly to book the rooms using the information you have to negotiate a rate equal to or lower than what you found online. I think it's going to be difficult to book 10 rooms at once online. You could do it in batches, but I think calling the hotel will get you a better rate.

Not sure if this helped, I've use http://www.motelgroups.com and have gotten pretty good prices, but all for hotels in the US, not sure if this is available for Germany though. Hotels submit bids to your requests and you can also negotiate with them for a lower price.

compare prices online first, then call the different hotels (sales manager) and ask what theyll do for you. 20 room nights when occupancy is low seems pretty reasonable.