Lo que quieres compartir:
// compartes el $user y la clave para encontrarlo es 'organization_account'
$this->addReference('organization_account', $user);Recuperas lo que has compartido y lo usas
$manager->merge($this->getReference('person_account'))Ejemplo completo:
public function load(ObjectManager $manager)
{
$userInGroup = new UserInGroup();
$userInGroup
->setGroup($manager->merge($this->getReference('GROUP_DEVELOP')))
->setRole($manager->merge($this->getReference('ROLE_USER')))
->setUser($manager->merge($this->getReference('person_account')));
$manager->persist($userInGroup);
$manager->flush();
$userInGroup = new UserInGroup();
$userInGroup
->setGroup($manager->merge($this->getReference('GROUP_DEVELOP')))
->setRole($manager->merge($this->getReference('ROLE_ADMIN')))
->setUser($manager->merge($this->getReference('organization_account')));
$manager->persist($userInGroup);
$manager->flush();
}