Database
[{_id:ObjectId("6314dc4de4ad4c8141ce0b08"),status:"started",channel:"myChannel",teams:[{name:"myFirstTeam",score:0,users:[{id:9082376,name:"myFirstUser"},{id:289168,name:"mySecondUser"},]},{name:"mySecondTeam",score:0,users:[{id:898323,name:"myThirdUser"}]}]}]
Query
db.collection.update({"channel":"myChannel","teams.users":{$size:1}},{$push:{"teams.$.users":{id:23424234,name:"myUserName"}}})