[{_id:1234567,"groupOfChats":[{0:{"idGroup":12345,"nameGroup":"someNameGroup","groupCreatorId":"someOtherId","membersId":["bar","foo","var"]},1:{"idGroup":12346,"nameGroup":"someNameGroup","groupCreatorId":"someOtherId","membersId":["var","some","other","thinks"]}}]}]

db.collection.aggregate([{"$match":{_id:1234567}},{"$set":{"groupOfChats.0.membersId":{"$reduce":{"input":"$groupOfChats.0.membersId","initialValue":["des","pa","cito"],"in":{"$concatArrays":["$$this","$$value"]}}}}},{"$set":{"groupOfChats.0":{$concatArrays:[{$slice:["$groupOfChats.0",1]},{$slice:["$groupOfChats.0",{$add:[1,1]},{$size:"$groupOfChats.0"}]}]}}}])