Most of you will arrive via Frankfurt Airport which is the largest international hub in Germany. From Frankfurt you may get onto a connecting flight to Hannover or take the ICE train which leaves directly from Frankfurt Airport.

More information at:
http://www.frankfurt-airport.de
http://www.lufthansa.com
http://www.bahn.de

From Hannover Airport (http://www.hannover-airport.de) and from Hannover Main Station there are regular trains and busses within the metropolitan region of Hannover. Your study-buddy will pick you up.
From Hannover to Clausthal there are regular trains and busses (approx. 2 hours distance). The Clausthal group will travel jointly from Hannover to Clausthal.