Is "jer" only singular? Duolingo did not accept among.
'Jer' is only plural.
So "mellem jer" could mean "among you" (when 'jer' refers to 3 or more people)?
Yes, I suppose so. Can you only use 'between' if you refer to two people?
I think there is a slight difference between 'between you' and 'among you'
For 'among you', I would use blandt or iblandt:
"Hvordan er kommunikationen blandt jer?"
Yes, when describing a position, between is used when 2 people or objects define the extremes, among when the position is defined by more than two people or things. So this looks like a good rule: between - mellen, among - blandt.