Sharepoint on Item Update add user

public override void ItemUpdated(SPItemEventProperties properties)
{
if (properties.AfterProperties["Status"].ToString().ToUpper() == "Closed".ToUpper())
{
SPSite site = new SPSite(@"http://SiteURL");
SPWeb web = site.AllWebs["mk"];
SPUser user = web.CurrentUser;
SPRoleDefinitionCollection roleDefinitions = web.RoleDefinitions;
SPRoleAssignmentCollection roleAssignments = web.RoleAssignments;
SPRoleAssignment roleAssignment = new SPRoleAssignment(@"mritunjay", "mritunjay@mritunjay.com", "Mritunjay", "myNotes");
SPRoleDefinitionBindingCollection roleDefBindings = roleAssignment.RoleDefinitionBindings;
roleDefBindings.Add(roleDefinitions["Full Control"]);
roleAssignments.Add(roleAssignment);
}
}