[{end_time:164,members:[{user_id:"1",left_at:73},{user_id:"2",left_at:0}]},{end_time:165,members:[{user_id:"3",left_at:74},{user_id:"4",left_at:0}]}]

db.collection.update({},[{$set:{members:{$concatArrays:[{$slice:["$members",{$subtract:[{$size:"$members"},1]}]},[{user_id:{$last:"$members.user_id"},left_at:"$end_time"}]]}}}],{multi:true})