[{"id":"room1","members":[{"id":"member1","acks":"XXX",},{"id":"member2","acks":"XXX",},{"id":"member3","acks":"XXX",},{"id":"member4","acks":"XXX",}]},{"id":"room2","members":[{"id":"memberA","acks":"XXX",},{"id":"memberB","acks":"XXX",},{"id":"memberC","acks":"XXX",},{"id":"memberD","acks":"XXX",},{"id":"memberE","acks":"XXX",},{"id":"memberF","acks":"XXX",}]},{"id":"room3","members":[{"id":"memberX","acks":"XXX",},{"id":"memberY","acks":"XXX",},{"id":"memberZ","acks":"XXX",}]}]

db.collection.aggregate({$match:{id:{$in:["room1","room2"]}}})