User.id => GroupUserRelation.user_id
GroupUserRelation.group_id => Group.id

public function getGroup()
{
  return $this->hasMany(Group::className(), ['id' => 'group_id'])
     ->viaTable(GroupUserRelation::tableName(), ['user_id' => 'id']);
}

public function getGroup()
{
  return $this->hasMany(Group::className(), ['id' => 'group_id'])
     ->viaTable('groupUserRelation');
}
public function getGroupUserRelation()
{
  return $this->hasMany(GroupUserRelation::tableName(), ['user_id' => 'id']);
}