db={"group":[{"_id":1,"groupName":"name1"},{"_id":2,"groupName":"name2"},{"_id":3,"groupName":"name3"}],"peoplegroups":[{"_id":10,"groupid":1,"user":"user1"},{"_id":11,"groupid":1,"user":"user2"},{"_id":12,"groupid":2,"user":"user3"}]}

db.group.aggregate([{"$lookup":{"from":"peoplegroups","localField":"_id","foreignField":"groupid","as":"User"}}])