Yes but...The Harlequin Book and formations have severe limitations.
Yes you can put them into your seer council but the only way you can have them is through either the Masque Detachment or the Cegorach's Revenge or to play Unbound.
Both the Masque detachment and Cegorach's revenge will easily cost you around 1000 pts.
roflmajog wrote:Yes they can join a seer council squad as harlequins and craftworlds are battle brothers and there are no specific rules preventing it.
Do note however that the shadowseer will not be casting on a 3+ as that bonus is for members of the formation, not members of the unit.
Indeed but the Shadowseer could protect the unit with it's Veil of Tears.