Freak parties are parties where 'freaks', i.e. people who are into vampires and experiencing biting, go to partake in their vice. The parties are always at different locations, presumably to keep the authorities, hate groups, and other intruders in the unaware.
When Anita visits a freak party in Guilty Pleasures it's held at someone's home and is little better than a pale prelude to an orgy even before any vampires arrive. It's heavily implied that the vampires are expected to be allowed to do whatever they want to whomever they want. Presumably, they self-police themselves enough that they don't kill or permanently damage people beyond some scarring (which seems to be considered a bonus) or the participants wouldn't be quite as enthusiastic about it and keep returning. Most of the humans present treat the whole experience as a kink and some, like Phillip, are entirely addicted to it, although he in particular is trying to kick the need. Despite the party location constantly changing, some especially violently inclined members of the Church of Eternal Life manage to raid the party that night, the human members carrying clubs and vampire members being weapons by themselves. When Anita confronts Malcolm about it, he agrees that the parties are an abomination and have to be stopped, but insists that it has to happen through legal means. Unfortunately for the so called freaks there's a faction of members who don't agree with Malcolm on that point, and he refuses to enforce his opinion.