[{"id":1,"activity_type":"view","user_id":1},{"id":2,"activity_type":"save","user_id":1},{"id":3,"activity_type":"save","user_id":1},{"id":4,"activity_type":"save","user_id":2}]

db.collection.aggregate([{$group:{_id:"$activity_type",count:{$sum:1},user_ids:{"$addToSet":"$user_id"}}},{$project:{_id:0,activity_type:"$_id",user_count:{$size:"$user_ids"},count:1}}])