If students are already part of a group, these groups can be manually created in Moodle by creating the groups and manually allocating the students to the groups. This option works well when you have a small number of students to allocate to groups. If your students are in a group and you have a large number of students the Programme Administrator can use the bulk enrolment method to create the groups.
You can quickly enrol participants on groups or edit their group membership from the Participants lLst.
This screencast demonstrates how to create groups in Moodle.