Not sure of a hack (see Update below) which affects only parties, but there is a cheat code you can enter while in Neighborhood view before you enter any of the lots:
intProp maxNumOfVisitingSims #
Replace # with the maximum number you want (don't make it too huge of a number, as that will slow or even crash your game).
This will affect all types of visitors, including parties and number of Sims on community lots.
If you want this enabled at all times, it can be added to your userstartup.cheat file and will thenceforth automatically be set for all neighborhoods.
UPDATE: Just now remembered that there is a party hack at Paladin's place (scroll down there a tad to get to it):
http://208.109.175.73/Sims-2-fixes-general.htm
