Could someone explain the use of the passive voice structure here? Is it used to communicate the middle voice? Or like a mutual reflexive thing?
You're right in that adding an "-s" at the end of a verb is a way to construct the passive voice in Norwegian. That is not what happens in this sentense. Adding an "-s" is also done for reflexive and reciprocal verbs. "Å møtes" is a reciprocal verb, there is a built in "each other".