[{"_id":"60d20d342c7951852a21s53a","site":"www.xyz.ie","A":["mary","jamie","john","mary","mary","john"]}]

db.collection.aggregate([{"$match":{"site":"www.xyz.ie"}},{$unwind:"$A"},{$group:{_id:{_id:"$_id",A:"$A"},site:{$first:"$site"},count:{$sum:1}}},{$sort:{count:-1}},{$group:{_id:"$_id._id",site:{$first:"$site"},A:{$push:"$_id.A"}}}])