You can easily move around by Metro, Tram or Taxi. I would not recommend the buses as is a bit more complicated to get around.

Metro ticket from the Airport costs 8 euros (I think) and can take you to Downtown Athens (Syntagma Station). In case you decide to stay close the Party location the you will need to change to Line 2 (red line in the map) direction Helliniko. You do not need to purchase another ticket but please ask this again at the ticket counter once you purchase your tickets at the Airport. Getting off at Helliniko, you can then take a taxi to your hotel.

Metro tickets costs 1,5 Euros regardless how many stations (unless you go to the Airport) and you can use the same ticket for up to 1,5 hours.

Athens Metro Map:

http://www.ametro.gr/page/default.asp?id=15&la=2

Athens Tram Map:

http://www.athens24.com/tram.html

Those who decide to stay close to the party location, they have also access to the Tram, which goes also to downtown Athens.

Taxis are relatively cheap in comparison to Germany. For example a ride from the party location to downtown Athens is approx. 15 Km and will cost max 20 Euros.