。
Come to the party with us.
Literal
Us [with-と] together party [to-に] go-[volitional-よう]-[insistence-よ].
一緒に ('together') is often paired with a と-marked companion: AとBが一緒に ('A with B together'). The volitional 行こう ('let's go') + よ makes this an invitation. パーティー is an English loanword. A natural, friendly invitation.