Explanation:

No online User

Now in the code behind create a DataView so later you can sort it according to LastLoginDate of the User.

public DataView GetAllUsersDataTable()
    {
        MembershipUserCollection mc = Membership.GetAllUsers();

        DataTable dt = new DataTable("LastUsers");
        dt.Columns.Add("UserName", typeof(string));
        dt.Columns.Add("Ldate", typeof(DateTime));
        foreach (MembershipUser st in mc)
        {
            if (st.IsOnline)
                dt.Rows.Add(new Object[] { st.UserName, st.LastLoginDate});
        }

        dt.AcceptChanges();
        DataView dv = new DataView(dt);
        return dv;
    }

On the Page Load event of the Page bind your Gridview to the created DataView.

DataView tView = GetAllUsersDataTable();
tView.Sort = "Ldate DESC";
GridView1.DataSource = tView;
GridView1.DataBind();

Download the Code: CheckOnlineUsers.zip