Check to verify if an e-mail address has been previously registered:

public bool LookupEmail(string EmailAddress)

{
 try
 {
  int intRecords = 0;

  ArrayList u = new ArrayList();

  u = UserController.GetUsersByEmail(this.PortalId, EmailAddress, 0, 1, ref intRecords);

  if (u.Count > 0)
  {
   return true;
  }
  else
  {
   return false;
  }  
 }
 catch (Exception ex)
 {
  Exceptions.ProcessModuleLoadException(this, ex);
  return false;
 }

}