using (myEntities ee = new myEntities())
{
GroupPost post = (from posts in ee.GroupPost.Include("SiteUser")
where posts.PostID == id
select posts).First<GroupPost>();
SiteUser u = ee.SiteUser.First<SiteUser>(e => e.UserName == HttpContext.User.Identity.Name);
post.SiteUser = u;
ee.SaveChanges();
}