Head to Tenpenny Tower to being this quest. Once the ghoul Roy leaves the intercom, use it and either pay caps or use a speech challenge to enter the tower. Once inside, speak with Chief Gustavo. He will ask you to kill Roy (who you just saw outside) who lives in the Warrington Tunnels. You can go ahead and kill Roy in the tunnels, or speak with him and instead agree to help him gain residence in the tower. Note that killing him is the only option to spare the lives of the people in the tower.
If you want to help Roy instead of killing him, you have two options. You can speak with Herbert Dashwood and convince him to give up his copy of the basement key, then use that to unlock the door to let the feral ghouls in to murder everyone. You will lose a ton of karma this way. Instead, you can talk with Tenpenny and ask him that if his residents are alright with the ghouls, would he allow them to live there. He agrees and you will need to speak with Edgar Wellington, Millicent Wellington, Anthony Ling, Lydia Montenegro and Susan Lancaster. You essentially bully them into agreeing, but once they've all been convinced, return to Tenpenny to finish the quest. Both of these options result in all the tower's residents being murdered, as even though the residents agreed to letting the ghouls in (for the second method), Roy kills them all anyway.
See Tenpenny Tower for more specific info.
Note: Make sure you do this quest before or at the same time as Strictly Business. During Strictly Business you will have to enslave Susan Lancaster. You can do this in private without the rest of the tower turning hostile, but if you are caught the other residents will try to kill you and you might not be able to finish the quest. You might also want to be working on You Gotta Shoot 'em in the Head at the same time, though it doesn't matter much since you can fast travel between locations.