Ornisso was visiting Formenos on some errand or another a couple weeks earlier, but no, he was not at the party. Methinks that Feanaro--knowing Feanaro--would not be likely to invite one of his half-brother's servants to his party, especially when he is keen on seducing said servant's ex-lover.

However, I agree that Eressetor would benefit from seeing that he's not missing out on much by not being with Ornisso. But you know Eressetor.... *sigh*