To Get the UserName from the ASP.NET membership tables using the UserID or Userkey the following snippet will work.

First add the Label Control in the .aspx page

<asp:Label ID="Label_UserInfo" runat="server" Text=""></asp:Label>

Now the snippet in the code behind, in the snippet if you have the UserID first we should convert into Guid

 /* Getting User Name from User ID*/

            Guid userKey = new Guid("5e5a79ca-321a-4158-ad8c-bdb42c5d02b4");

            MembershipUser UserInfo = Membership.GetUser(userKey);

            Label_UserInfo.Text += UserInfo.UserName;

To use the MembershipUser you should have System.Web.Security Namespace

using System.Web.Security;