PUGs are sorted based on your ESEA Rank. The balance of your PUG greatly depends on the volume of players also queuing at the time when you enter the queue. We create the best possible PUG with the players available from the queue so that you have the most competitive match possible at the time. We match games based on a number of factors that will affect how players enjoy the game such as the region of the participants, the size of the groups, their ping to our servers and we try to keep players who are on each other's block list to separate teams.
Eventually, it will start to widen the gap (to a degree) between ranks so that the queue time remains reasonable and that a PUG can be played. As an example, it is not unusual to be rank A- and have teammates that are B or B+.