[{_id:ObjectId("62015250cb18d3cadb32c38e"),user:{pk:"7485070525"},usertags:[{user:{pk:"1234"}}]},{_id:ObjectId("62015250cb18d3cadb32c38f"),user:{pk:"334777488"},usertags:[{user:{pk:"334777488"}},{user:{pk:"271738610"}},{user:{pk:"31961021"}}]}]

db.collection.aggregate([{$group:{_id:null,users:{$push:"$user.pk"},usertags:{$push:"$usertags.user.pk"}}},{$project:{users:{$setUnion:["$users",{$reduce:{input:"$usertags",initialValue:[],in:{$concatArrays:["$$value","$$this"]}}}]}}}])