Multi-Output Recommender: Items, Groups and Friends, and Their Mutual   Contributing Effects
  Due to the development of social media technology, it becomes easier for users to gather together to form groups. Take the Last.fm for example, users can join groups they may be interested where they can share their loved songs and discuss topics about songs and singers. However, the number of groups grows over time, users need effective groups recommendations in order to meet more like-minded users. 
Wei Zeng and Li Chen
cs.IR