To unlock the House Party trophy in Assassin's Creed III Remastered, you will first need to progress through the game until the fourth memory in Sequence 5 called River Rescue.
Once you complete the mission, you will be shown a tutorial for a new type of side mission that is now available. Move towards the new mission icon that becomes available. When you arrive there, you will have to kill four enemies. When they have been dealt with, interact with the rope on the rock to free a prisoner. This will trigger a cutscene, after which the trophy will unlock.