The Pensions Act 2008 says that directors are not treated as workers if they're not employed under a contract of employment to work for the company AND there is no one else employed by the company under a contract of employment.
In other words, a company with just one-director and no other employees is exempt from auto enrolment. What this means in practice is that they don't have to complete a declaration of compliance or set up a pension scheme.
It's worth noting however that some one-director companies may still get a staging date letter from The Pensions Regulator (TPR)
Where there is more than one director in a company, each director should be looked at separately to determine worker status.
But regardless of the status of the directors, if the company takes on an employee in the future, the chances are that the new employee will be a worker, and if any of the directors are employed under a contract of employment, they will be workers too.
What about office-holders?
Office-holders include non-executive directors, company secretaries, board members of statutory bodies and trustees. They normally won't have a contract to work for the company concerned so normally won't be treated as workers.
However they might also have a contract for services for part of their duties, so could be treated as a worker in respect of that contract.