> use admin
switched to db admin
> db.addUser('neo','chen')
{
        "user" : "neo",
        "readOnly" : false,
        "pwd" : "68ace374737253d87e0ec91d4fcb673d"
}

> db.system.users.find()
{ "_id" : ObjectId("4c481404b9db6474d2fcb76f"), "user" : "neo", "readOnly" : false, "pwd" : "68ace374737253d87e0ec91d4fcb673d" }

> db.auth('neo','chen')
1