Can an owner be prevented from having a party that included outside guests coming into the community?

Yes. A board can absolutely pass a rule or update an existing rule to address the use of common areas such as the pool or clubhouse for private social events hosted by residents. That rule might limit the number of such events or the number of people that can attend or completely close down access to the common area meeting rooms or even keep track of how many guests are entering a unit in a specific time frame.