[{"_id":{"$oid":"5ff09030cd55d6d9f378d460"},"username":"a value","userid":"123456","last_access_ts":1612426253,"last_access":"2021-2-4 9:10:53","anotherid":12345678910,"verified_date":"2021-1-2 16:24:32","verified_ts":1609601072,"group_users":{"-1001159747589":[{"anotherid":12345678910,"userid":"123456"}],"-1001143137644":[{"anotherid":12345678910,"userid":"123456"}],"-1001368608972":[{"anotherid":12345678910,"userid":"123456"}]},"registered_access":"2021-1-2 16:24:42",}]

db.collection.aggregate([{$match:{username:"a value"}},{$project:{_id:1,userid:1,groups:{$let:{vars:{group_arr:{$objectToArray:"$group_users"}},in:{$arrayToObject:{$map:{input:"$$group_arr",in:{k:"$$this.k",v:{$size:"$$this.v"}}}}}}}}}])