ADO.NET :: Binary Tree - Want To Check All The Records?

I m creating a project in which only two members can register by using one perticular ID as a Sponser ID. I am using VS 2008 with C# language and LINQ as Query. At the point of insertion for register entries, i invoke a method BINARYCHECK which contains LINQ query. I passed _txtSponserId as a parameter to the BINARYCHECK method. Now in the function definition section i have written LINQ query which checks in database that if particular sponser ID is bieng used maximum of two times by any other user. But i only can check one record in the database. I want to check for all the records. Below is my code:

var a =UserBl.BinaryCheck(_txtSponserId);//function call

public static List<tblUser> BinaryCheck(string s)

{[code]....

This code doesnt check for how many times SponserID is Used. This is why i get i=1 and return data even if SponserId is used only One time. On the contraty i want to implement that if SponserId is used only once in database then function should return null other wise it shold return 1. How to do this?