Considering your ddbb.json configuration file is something like this:

{
  "public" : {
    "host"     : "localhost",
    "port"     : 27017,
    "database" : "culture",
    "readonly" : {
      "username" : "culture",
      "password" : "culture"
    },
    "full" : {
      "username" : "privateuser1",
      "password" : "secret1"
    }
  },
  "private" : {
    "host"     : "localhost",
    "port"     : 27017,
    "database" : "private",
    "username" : "privateuser2",
    "password" : "secret2"
  }
}

You should perform this commands in order to create users with the correct
privileges over the MongoDB database:

> mongo

> use culture
> db.addUser("culture", "culture", true)
> db.addUser("privateuser1", "secret1")
> use private
> db.addUser("privateuser2", "secret2")