Overview
On our community website we are placing the mini badge in every activity stream item which help us distinguish between users with active, expired, or no license at all.
In our case, the little green badge in lower right-hand corner JS Active help us determine that user is having an active license, so our support team can react accordingly.
In other use cases, you might want to have a VIP users, or simply "tag" the people that belong to a different Joomla User Group like this (notice the 
badge under avatar)
Template Override Is The Way To Go
While Joomla users are accustomed to use plugins to change the certain behavior of Joomla functionality, in this particular case it would be very hard to do it with a plugin because every Joomla site have different user groups, with the different unique ID which in turn would cause plugin to display nothing and give impression that it "don't work".