cannot do asqueryable on mongodb collection

I got two models, a User and a Team as below:

    [BsonRepresentation(BsonType.ObjectId)]
    public ObjectId _id { get; set; }
    [Display(Name = "Password:")]
    public string Password { get; set; }
    [Display(Name = "Confirm:")]
    public string ConfirmPassword { get; set; }
    [Display(Name = "Email:")]
    public string Email { get; set; }
    [Display(Name = "Username:")]
    public string UserName { get; set; }
    [Display(Name = "Firtname:")]
    public string Firstname { get; set; }
    [Display(Name = "Lastname:")]
    public string Lastname { get; set; }
    [Display(Name = "Country:")]
    public string Country { get; set; }
    [Display(Name = "City:")]
    public string City { get; set; }
    [Display(Name = "Birthdate:")]
    public int Birthdate { get; set; }
    public List<Team> Teams { get; set; }


   [BsonRepresentation(BsonType.ObjectId)]
    public ObjectId TeamID { get; set; }
    public string TeamName { get; set; }
    public string UserName { get; set; }
    public int LeagueID { get; set; }

    public List<Player> Player { get; set; }

So I've created a user but now I want to add teams to my user. This is the code I'm using:

  var databaseClient = new MongoClient(Settings.Default.FantasySportsConnectionString);
  var server = databaseClient.GetServer();
  var database = server.GetDatabase("Users");
  var collection = database.GetCollection<User>("users");

  var user = collection.AsQueryable().First(o => o._id == Session["ID"]);

  user.Teams.Add(new Team { TeamID = new ObjectId(), TeamName = "Some Team" });

But when I do this I get these errors:

1: Instance argument: cannot convert from 'MongoDB.Driver.MongoCollection<SportsFantasy_2._0.Models.User>' to 'System.Collections.IEnumerable'

2: 'MongoDB.Driver.MongoCollection<SportsFantasy_2._0.Models.User>' does not contain a definition for 'AsQueryable' and the best extension method overload 'System.Linq.Queryable.AsQueryable(System.Collections.IEnumerable)' has some invalid arguments