Finding the User Name of the currency logged on user or any user is very simple.
You can use System.Web.Security.Membership namespace.To find the user name of the current logged in user.
var username = User.Identity.Name;
Further more you can get all membership details of any registered member by
var username = User.Identity.Name; var membershipUser = Membership.GetUser(username);// pass in the username of the user whose membership details you want to access.
Now to access the Guid of the user you can do this.
if (membershipUser != null){ var userGuid = (Guid)membershipUser.ProviderUserKey; }