I propose that when a group of people upvote each other repeatedly, they get awarded the "Circlejerk" badge.
a group of people upvote each other repeatedly
So... what reason allows this to be within our standards? What this means that there is now a badge that supports:
Other types of illegal group upvoting
We already have badges for voting and upvoting already. What good does this do? Groups of people will dive for the badge, give themselves lots of rep they only received because of the badge. And serial upvoters will have a reason to avoid suspension. This will only create chaos and will bring trouble-makers to Stack Exchange.
And anyway, the proposed badge name is "Circlejerk". You are right. Those people are in their little circles/groups and are jerks. This behavior will never be tolerated and in response, we give them a gift or two: The Amazing Suspension and The Destruction of the Violator's Account(s). So don't even think about this type of serial upvoting to become legal at all.