Order of the Phoenix
Order of the Phoenix is the name given to the organisation founded by those resisting You-Know-Who and his followers during the First Wizarding War. Original members included Lily and James Potter, The Longbottoms, Sirius Black, Remus Lupin and Albus Dumbledore. After Voldemort disappeared, the Order was disbanded, but was later revived before the Second Wizarding War when rumors began circulating about Voldemort’s return.
The headquarters of the Order are at 12 Grimmauld Place in London, which is the Black family home. Only those who have been taken there by a member of the Order can see the building, to everyone else, it simply isn’t there.