So this lesson suddenly starts to use quite some s-verbs, but I'm not sure I understand the difference. If I understand correctly both å møte and å møtes mean "to meet" so what's the difference?!? Why use møtes here?!?
Am I right (I have read all the links), that møter is used when i.e. I meet somebody (on the street), and møtes is like meeting each other?
I am not a native speaker, but based on the links I believe you are correct. So here it means that they meet each other (reflexive). You could not say, however, "Han møtes henne på museet" because that's not reflexive.
Is this a possible translation? "They are being met at the museum." How to tell if an "s" verb is passive or reciprocal?
It would be very rare to use the passive form without an object; someone they were being met by. Even then, there would be a preference for either making that someone the subject, or using "blir(/vil bli) møtt" rather than "møtes".
"Guiden møter dem på muséet."
"De vil bli møtt av en guide på muséet."