MDL-63485 core_user: Allow filtering by No groups on participants page

A new optional parameter $context has been added to the
core_group::groups_get_members_join() function.
Besides, some core_group methods now accept -1 (USERSWITHOUTHGROUP) for
the groupid field.

Backport of MDL-61967.

lib/tests/accesslib_test.php

@@ -2335,6 +2335,40 @@ class core_accesslib_testcase extends advanced_testcase {
 
     }
 
+    /**
+     * Test that enrolled users SQL does not return any values for users
+     * without a group when $context is not a valid course context.
+     */
+    public function test_get_enrolled_sql_userswithoutgroup() {
+        global $DB;
+
+        $this->resetAfterTest();
+
+        $systemcontext = context_system::instance();
+        $course = $this->getDataGenerator()->create_course();
+        $coursecontext = context_course::instance($course->id);
+        $user1 = $this->getDataGenerator()->create_user();
+        $user2 = $this->getDataGenerator()->create_user();
+
+        $this->getDataGenerator()->enrol_user($user1->id, $course->id);
+        $this->getDataGenerator()->enrol_user($user2->id, $course->id);
+
+        $group = $this->getDataGenerator()->create_group(array('courseid' => $course->id));
+        groups_add_member($group, $user1);
+
+        $enrolled   = get_enrolled_users($coursecontext);
+        $this->assertCount(2, $enrolled);
+
+        // Get users without any group on the course context.
+        $enrolledwithoutgroup = get_enrolled_users($coursecontext, '', USERSWITHOUTGROUP);
+        $this->assertCount(1, $enrolledwithoutgroup);
+        $this->assertFalse(isset($enrolledwithoutgroup[$user1->id]));
+
+        // Get users without any group on the system context (it should throw an exception).
+        $this->expectException('coding_exception');
+        get_enrolled_users($systemcontext, '', USERSWITHOUTGROUP);
+    }
+
     public function get_enrolled_sql_provider() {
         return array(
             array(