Groups, under the course administration panel, and group submission settings allow an instructor to assign students to groups for individual activities. Because students can be in multiple groups at once, instructors can set up different group memberships for different activities (e.g., using larger groups for forum discussions, and smaller groups for peer review activities). In addition, groups of students can each be assigned separate activities.
Course Administration Menu > More > Users > Groups
Moodle How-To: Groups > Auto Grouping
Moodle How-To > Groups > Manual Grouping
Moodle How-To > Groups > Assignment Settings