[{"_id":1,"some_field":"some_value","array":[{"group":"a","subgroup":"aa","value":1},{"group":"b","subgroup":"bb","value":2},{"group":"a","subgroup":"ab","value":2}]}]

db.collection.aggregate([{"$match":{"_id":1}},{"$unwind":"$array"},{"$project":{"array":{"group":"$array.group","values":"$array"},"some_field":1,"my_id":"$_id"}},{"$unset":"array.values.group"},{"$group":{"_id":"$array.group","values":{"$push":"$array.values"},"some_field":{"$first":"$some_field"},"my_id":{"$first":"$my_id"}}},{"$set":{"array":{"group":"$_id","values":"$values"}}},{"$unset":"values"},{"$group":{"_id":"$my_id","array":{"$push":"$array"},"some_field":{"$first":"$some_field"}}}])