International HI Group Hostels 

HI Iceland is a not-for-profit organisation and part of Hostelling International, the world's largest youth travel organisation. HI's mission is to provide opportunity for all, but especially young people, for education through travel. The organisation has a strong commitment to sustainability.  
HI has some great group hostels in countries all around the globe – check out the opportunities here.
A Group Membership provides a range of benefits. Full details are here.