The House on the Edge of the Park

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Alex is a darkly sinister thug driving around New York City at night when he spots a young woman, Susan, driving alongside him. He follows her to a nearby park where he cuts her off, gets out of his car, and runs into hers where he throws her into the back seat and proceeds to savagely rape her before strangling her to death. He takes her locket as a trophy to another one of his many killings.
Some time later, serial killer/rapist Alex is with his working-class friend Ricky, both of whom work in a local mechanic garage which is closing up for the evening. Alex is planning to go 'boogying' at a local disco and Ricky is tagging along. Suddenly, a large Cadillac vehicle pulls into the underground garage which contains a young, well-dressed yuppie couple named Tom and his girlfriend Lisa where they ask for assistance for their car's motor is making strange sounds. Alex refuses to help them saying that the garage is closed, and continues to refuse assistance even after Tom offers him money. But the slow-witted Ricky decides to help the couple and upon checking under the hood, finds a loose wire on the alternator and fixes it. Grateful, Tom tells Alex and Ricky that they are driving to a friend's house in New Jersey for a party, and Alex asks them if he and Ricky could tag along, and Tom agrees. Before closing up the garage for the night, Alex stops by his locker which is filled with various weapons he uses to kill people. Alex picks out just one; a straight-edged razor, before closing up and getting into Tom and Lisa's car with Ricky for the drive to New Jersey.
A little later, the four arrive at a large villa situated next to a large, dark, empty park where they are welcomed by the home's owner Gloria, and are introduced to her friends Glenda and Howard. But minutes after Alex and Ricky's arrival, there are signs of tension as it becomes obvious that the rich people are looking for kicks, as are Alex and Ricky. It first starts when Ricky is forced to make a fool of himself when Gloria asks him to do a striptease to some disco music, while he is further humiliated by forced to drink some hard liquor with each move he makes. But Alex stops Ricky before he can take it all off.
While Tom, Howard, and Glenda play a card game of poker with Ricky, which is obviously rigged, Lisa begins sexually teasing Alex with her come-ons, and goes further to invite him to an upstairs bathroom to shower with her, only to push him away. As Alex grows more frustrated and angry, he later sees that the hosts are cheating at playing cards with Ricky. Finally, Alex pulls out the razor he has, and a fight breaks out between him and Howard. Alex throws Howard outside the back door where he endures a vicious beating and thrown into the backyard swimming pool, and further urinated upon by the laughing Alex, who drags Howard back inside and ties him to a piano leg, while he proclaims that he is running the party now.
At this point, the film descends into an unrelenting catalog of abuse and humiliation as Alex and Ricky further beat on their hosts-turned-hostages with Alex slashing Tom's face with the razor, and beating his face into the poker table. Ricky holds the others at bay with a wine bottle and Alex corners Gloria and sexually fondles her. Lisa runs to an upstairs bedroom where she tries to escape, but Alex catches her and proceeds to rape her, while the soundtrack plays the romantic theme song "Sweetly".
When Alex takes Lisa downstairs to rejoin the other hostages, the front doorbell rings. Alex forces Gloria to answer and it's a neighbor, Cindy, a local teenage girl. Alex grabs Cindy, while Gloria takes advantage of this to try to escape. Ricky, still holding the broken wine bottle, runs after her and catches up with Gloria on the perimeter of the grounds. Ricky shows Gloria that he means her no harm by throwing away the wine bottle. Gloria responds to his simple nature by stripping off her clothes and seducing him.
Meanwhile, back at the house, Alex cuts Cindy's blouse off with the razor while singing. Ricky then returns to the house with Gloria just as Alex forces Cindy to strip off the rest of her clothes, and proceeds to savagely slash her nude body with his razor over and over again. At this point, Ricky realizes that things are getting out of hand and attempts to stop Alex. Upset at being "betrayed" by his friend, Alex turns against Ricky and slashes his abdomen wide open, and then breaks down at what a mess he made of Ricky.
Taking advantage of this distraction, the bloodied and battered Tom runs into the nearby study and pulls out a 9mm pistol out from a desk drawer... and the tables are instantly turned. Tom shoots Alex a few times, then kicks him back out the glass back door and into the back yard. Gloria and the other women untie Howard, and the five hosts descend upon the fatally wounded Alex lying on the ground. Tom removes the locket Alex is wearing and reveals the reason for all this: the woman that Alex raped and murdered in the opening scene was Tom's sister and Tom wanted revenge, so he and Lisa wanted to lure them to Gloria's house so they can kill them and make it look like self-defense. Tom shoots the wounded Alex in the groin, which causes him to fall into the backyard swimming pool after shouting on-screen for at least 15 seconds, in extreme slow-motion. Tom and Lisa take turns shooting Alex, who thrashes weakly in the water, before a final shot to the head by Howard finishes him for good.
Returning to the house, Howard wants to shoot Ricky, but Gloria stops him. Glenda tends to the wounded Cindy. Tom and Lisa go into the study where they talk about this plan of revenge they had and if they went too far for it to work. Tom says that despite some mistakes and inconsistencies, that it worked out for the best, and then picks up the phone to call the police.

Question: Who does Lisa invite upstairs with her?
Answer: Lisa invites Alex upstairs