I saw this movie at least 10-15 years ago on TV.
As far as I can remember there is a monster in a house, I think a party was going on. Well the monster can absorb and look like people it absorbs. Some kids and teens are the main characters trying to stop the monster. At one point they think it is friendly and they are feeding it hot dogs but the thing got spooked and decided to absorb the teen guy who was if I remember dressed like a punk guitarist.
At the end of the movie it turns out the monster was a pet from a different planet and when the alien came to retrieve it and help, the kids cracked its space helmet thinking it was a bad alien and before the scene cuts they are all getting sucked in by the pet monster. It ends with the monster getting in a cab or car, and I think the driver makes some joke about a Halloween party gotten out of hand, not knowing the thing in his car is the monster.
Hope this give some clues because ever since I saw it as a kid I always wanted to watch it again for fun.
P.S. I think the monster looked like Jabba the Hutt ish when it was failing at copying an entire human's looks.