now the groups selection works.

frontend/src/components/student_list/StudentList.vue

@@ -290,10 +290,10 @@ export default {
       }
     },
     async getUser (student) {
-      this.user = await fetchUser(student.pk)
+      this.user = await fetchUser(student.userPk)
     },
     setExerciseGroups (groups, student) {
-      getUser(student).then(() => {
+      this.getUser(student).then(() => {
         setGroups(this.user.pk, groups)
       })
     },