[{"_id":1,"entities":{"user_mentions":[{"name":"user_name","screen_name":"user_screen_name"}]},"user":{"name":"Author Name","screen_name":"Author Screen Name"}},{"_id":2,"entities":{"user_mentions":[{"name":"user_name","screen_name":"user_screen_name"}]},"user":{"name":"Other Author Name","screen_name":"Other Author Screen Name"}}]

db.collection.aggregate([{"$project":{"mentions":"$entities.user_mentions.screen_name","author":"$user.screen_name"}},{"$unwind":"$mentions"},{"$group":{"_id":{aut:"$author",ment:"$mentions"},"count":{"$sum":1},author:{"$first":"$author"},mentions:{"$first":"$mentions"}}},{"$project":{_id:0}}])