The more people you can house, the larger the mess you will be tasked with cleaning. And... The more people you can house, the more noise complaints you can expect from your neighbors.
Pick a bed size that matches the room. Will a King take up too much floor space, and make it feel cramped, or will a Queen look dwarfed in that cavernous space? Two Twins in one room would work well when you've got a group that includes two people who wouldn't necessarily feel comfortable sleeping in the same bed together... Two adult siblings are going to want separate beds, which means that when Mom and Dad are sleeping in one bed, Grandma and Grandpa in another and all that's left is a room with a queen... Sister is sleeping in the bed... her brother is sleeping on the couch... and those strange snags in the upholstery mean he probably slept in his street clothes, and didn't remove his belt. (Sure... you could get a sofa that converts into a bed, but that just means that they're going to want to bring two more people with them. Eight people in a two bath house... How big is your water heater?)