$this->frontendUserGroupRepository = $this->objectManager->get('TYPO3\CMS\Extbase\Domain\Repository\FrontendUserGroupRepository');
$usergroupObj = $this->frontendUserGroupRepository->findByUid(1);

config.tx_extbase.persistence.classes.TYPO3CMSExtbaseDomainModelFrontendUserGroup.mapping.tableName =  fe_groups