Even if you know the majority of people at a party, you’re bound to run into someone you forgot existed. If you want your memory to seem better than it is, just check Facebook.
If the party was organized via a Facebook invite, you can just check the attendee list and look at everyone’s picture. That way you can put faces to names in advance so you don’t seem so forgetful. Even if that isn’t an option, however, you can just check the Facebook pages of friends you know are attending. Just sort their friends lists so it only shows local people and take a quick glance. If you took the thorough approach you’d be combing lists all day long, so don’t do that. Just have a quick refresher and keep an eye out for people you commonly forget. If you do this enough, eventually you won’t forget them!