controller.php

class SomeController extends ... {

	/**
	 * @var \TYPO3\FLOW3\Security\Context
	 * @FLOW3\Inject
	 */
	protected $securityContext;

	public function someAction() {
		$roles = $this->securityContext->getRoles();
	}
}

Repository

class SomeRepository extends \TYPO3\FLOW3\Security\AccountRepository {

	/**
	 * @param \TYPO3\FLOW3\Security\Policy\Role $role
	 * @return array
	 */
	public function findByRole(\TYPO3\FLOW3\Security\Policy\Role $role) {

//		$query = $this->createQuery();
//		return $query->matching($query->in('roles', array((string)$role)))->execute();

		$result = array();
		$accounts = $this->findAll();
		foreach ($accounts as $account) {
			if ($account->hasRole($role)) {
				$result[] = $account;
			}
		}
		return $result;
	}
}

Hm, that in() should work, but in the newsgroup someone asked about it just these days… we should try it and if it does not work, that's a bug.