My first party was very structured. There were only eight guests, two for each house. And there was a proper Hogwarts feast. To ensure each student sat with their respective housemate, I purchased inexpensive taupe table runners and several lengths of braided trim in each of the primary house colors. I simply hot glued the trim to the edges of each runner and voila!
As a cute extra detail, I made little drink markers for everyone's goblets by attaching celestial charms to floral wire and curling the wire around a lipstick tube!One detail that had me stressing out was the house points. In the books they are described as being giant hour glasses filled with colored gems. Well even a small approximation of such a thing was going to be impossible to find. So I ended up getting a set of four hanging vases and bags of colored squashed marbles instead. Not perfect, but it did the job. I was also given a set of four plaques that had startlingly appropriate shields on each for the four houses. A lion for Gryffindor, a two-headed bird for Ravenclaw, a strange serpent for Slytherin, and a castle with medal for Hufflepuff (I decided this could be a symbol of loyalty since there was no badger plaque).