This may be intentional, especially if the guests' needs are not being met. Are you seeing any telltale thought bubbles above their heads before they try to leave that may indicate boredom, hunger, low energy, etc.? The inactive sims at Uni World are notorious for not being able to take care of their own needs consistently, so they may have arrived to the party that way or they may need some coaxing to join in any activities you may have planned for them if they aren't going to find their way to the food, music for dancing, the pool, or each other to socialize (assuming they don't actually hate the other guests) by themselves. Is there entertainment going on? Can you chat them up a bit as they arrive and bring them each, or as many as you can reach, a drink or a plate of food?
If you are using mods and have NRaas MasterController, you can get a reading on their motives (needs), if that helps figure it out.
