I was able to add "$user - > username" to the attendance table through this process :
- in renderer.php (moodle\mod\attendance\renderer.php) :
1- at line approx 645:
change : return $fullnamehead;
to : return ' Username / '.$fullnamehead;
2-at line approx 950:
change : $row->cells[] = $this->user_picture($user);
to : $row->cells[] = '<p style="float:left;margin-top:10%;">'.$user->username.' </p>'.$this->user_picture($user);
3-at line approx 547:
add : $fullname = $user->username.' '.$fullname;
under : $fullname = $this->user_picture($user).$fullname;
this should add the column username in the session table & the report table.
tested on moodle v3.1.
hope it helps