How, because they have an academy and they applied (seems that was the only criteria) and as for why, after seeing their group of
Chelsea, Crystal Palace, Fulham, Liverpool, Manchester City, Middlesbrough, Wolverhampton Wanderers
I am sure they are asking the same thing as only the top 2 progress to the main knock out stages.
Cant quite see why they are allowing 3 overage players and one overage GK though if its the U21 league to give the young players a game unless there is no Premier League reserve league