[{_id:"1",activity:"movie",attendees:[{firsName:"personA",lastName:"lasty"},{firsName:"personB",lastName:"namey"},{firsName:"personC",lastName:"blabla"}]},{_id:"2",activity:"bowling",attendees:[{firsName:"personA",lastName:"lasty"}]}]

db.collection.aggregate([{"$replaceRoot":{"newRoot":{"$mergeObjects":["$$ROOT",{"$arrayToObject":{"$reduce":{"input":"$attendees","initialValue":[],"in":{"$concatArrays":["$$value",[{k:{"$concat":["attendees_",{"$toString":{"$add":[{"$indexOfArray":["$attendees","$$this"]},1]}}]},v:"$$this"}]]}}}}]}}}])