[{"sender":"bob@gmail.com","to":["harry@gmail.com","tom@gmail.com"],"text":"Lorem ipsum dolor sit amet, consectetur adipiscing elit."},{"sender":"bob@gmail.com","to":["tom@gmail.com"],"text":"Lorem ipsum dolor sit amet, consectetur adipiscing elit."},{"sender":"harry@gmail.com","to":["bob@gmail.com"],"text":"Lorem ipsum dolor sit amet, consectetur adipiscing elit."}]

db.collection.aggregate([{$facet:{totalSent:[{$group:{_id:"$sender",totalSent:{$sum:1}}}],totalReceived:[{$unwind:"$to"},{$group:{_id:"$to",totalReceived:{$sum:1}}}]}},{$project:{all:{$concatArrays:["$totalSent","$totalReceived"]}}},{$unwind:"$all"},{$group:{_id:"$all._id",totalSent:{$sum:"$all.totalSent"},totalReceived:{$sum:"$all.totalReceived"}}}])