[{"instanceId":"12","eventId":"0-1b","activityType":"A","personId":"1",},{"instanceId":"12","eventId":"0-1b","activityType":"B","personId":"2",},{"instanceId":"13","eventId":"0-1c","activityType":"B","personId":"2",},{"instanceId":"14","eventId":"0-1d","activityType":"C","personId":"3",},{"instanceId":"14","eventId":"0-1d","activityType":"C","personId":"4",}]

db.collection.aggregate([{$group:{_id:"$activityType",personIds:{$push:"$personId"},instanceIds:{$push:"$instanceId"}}},{$project:{_id:0,type:"$_id",peopleCount:{$size:"$personIds"},instanceCount:{$size:"$instanceIds"}}}])