Computer Supported Collaborative Work is an exciting research field. The original research themes have been changing in time and new areas have appeared in recent years. However, research has always been close to applications. The Collaboration Researchers' International Workshop (CRIWG) has been held every year since 1995. This conference is special in various aspects, including the provision of ample time to present each paper, so that the issues can be discussed in depth. That feature has attracted authors who want to receive significant feedback to continue their research work. The 21st version of CRIWG was held in Yerevan, Armenia, September 22-25, 2015. Following a tradition, the authors of the best papers of the conference were invited to submit significantly extended, improved versions of their contributions to be considered for a journal special issue. For this version of the conference, the accepted papers would be published in a special issue of J.UCS, as we had done for another version of the conference in the past. Furthermore, there was a J.UCS open call so that any author could also submit papers on the "Computer Supported Collaborative Work" subject. A total of 11 papers - including invited and submitted after the open call - were rigorously reviewed by three qualified reviewers each and 8 papers passed this first round. All selected papers have been revised by their authors and subject to a new review round, which in some cases meant new changes by the authors. The final papers are presented in this issue.