Fill List Wcf serviceParameter do not have Add method

Hi,

I have a a WCF service like this

 [OperationContract]
void Insert(List<PersonList> person);
 [DataContract]
    public class PersonList
    {
        [DataMember]
        public List<Person> Persons{ get; set; }

    }
     [DataContract]
     public class Person
     {
         [DataMember]
         public  string Name
     }

From the client I want to Fill PersonList, I have

PersonList per = new PersonList();
Person pp = new person();
pp.Name = "aaa";
per.Persons.Add(pp);

.ADD Method Does not Exists?
What I am missing?

Diego