Definition
In projects that use Agile methods (like Scrum) for example, making architectural design decisions regarding software is the responsibility of the entire development team as a group, instead of a single person. Current techniques usually focus on the identification of quality attributes , meaning that the quality of decisions directly reflects the quality of the software architecture. A group is usually a 10 member team, that develops software in sprints.
Further Reading
For more on Group Decision Making, read this.