Well, Regulus was one of Slughorn's Slytherins and it doesn't run contrary to canon to say that he was in the Slug Club or attended some of Slughorn's parties. Harry notices the resemblance to Sirius, or Slughorn happens to be naming people in a picture with Lily and we get a quick explanation that Regulus was Sirius' brother and died young at Voldemort's hands - which sets up his involvement in DH in an easy ten seconds.