、。
He invited me to go to the party together.
Literal
Party [to-に] together go-[let's-う] [quot-と], he [by-に] was-invited [polite-ました].
A passive sentence: the speaker is the implicit topic (receiver of the invitation), 彼に marks the agent ('by him'). The quoted portion 一緒に行こう is an intentional / volitional form ('let's go together') framed by と — Japanese uses quoted volitionals to report the content of an invitation rather than introducing a subordinate 'that' clause. This [volitional]-と construction is the standard way to report invitations and stated intentions.