Those gathered — for celebration, or as a side in an exchange; the line where both meanings meet is consent.
Latin partita (divided), from partiri (to divide, share out), from pars (a part), via Old French partie; those gathered as one side of a division — whether in celebration or in an exchange.
A gathering, or a side named in an agreement — the same word carries both because getting people to one table depends on the same thing an exchange does: everyone's actual consent.