You can obtain ID of each live user if you are using Live SDK. Here's code for you.
private async Task<string> GetLiveUserId()
{
string ID = "";
var auth = new LiveAuthClient();
var loginResult = await auth.LoginAsync(new string[] { "wl.signin", "wl.basic" });
if (loginResult.Status == LiveConnectSessionStatus.Connected)
{
var liveClient = new LiveConnectClient(loginResult.Session);
var myData = await liveClient.GetAsync("me");
ID = myData.Result["id"].ToString();
}
return ID;
}