Any Craftworld Eldar, Dark Eldar or Exodites army may include units of Harlequins. All Harlequin units count as Elites in this case, regardless of which category they are normally taken from. Any other army (except Chaos) that is fighting against a Chaos army may take one unit of from the following army list as an Elites choice. This does include such strange races as the Necrons and Tyranids – think of it as the Harlequins launching an attack at the same time as their 'allies' rather than being allied with them in any true sense.
Alternatively, a Craftworld Eldar, Dark Eldar or Exodites army can take an allied detachment of Harlequins (or the Harlequins can take the allies, depending on which way around you want to look at it… ). The rules for detachments can be found on page 131 of the Warhammer 40,000 book.
In both of these circumstances its more preferable for the Harlequins to be controlled by a different player, just for the increased confusion and quarrel factor if nothing else!