Scraping HTML via async controller & classes + HTML agility pack

public async Task<ActionResult> UpdateHandicap()

We add the Async suffix to methods that are marked with the async keyword. This is a naming convention that as you'll see in a moment Entity Framework follows too.

var results = db.Users.ToList();

You can use the Users directly in the loop, you don't have to call ToList first.

foreach (Users user in results)
{
    user.Index = await Calculations.GrabIndexValue(user.GHID);
    db.Entry(user).State = EntityState.Modified;
    db.SaveChanges();
}

The async calls are incomplete. SaveChanges would still block so you also want to use the

await db.SaveChangesAsync();

but do you really need to call SaveChanges in a loop? This could be bad for the performance. I think you should do it after the loop:

foreach (var user in db.Users)
{
    user.Index = await Calculations.GrabIndexValue(user.GHID);
    db.Entry(user).State = EntityState.Modified;
}
await db.SaveChangesAsync();

Don't Try if you aren't going to do something with what you catch, I didn't think you were able to write a catch block without a parameter anyway.

Then you are going to want to change that ugly if statement and make it much cleaner with a ternary Statement

This:

    try
    { 
        index = htmlDoc.DocumentNode.SelectNodes("//*[@id=\"ctl00_bodyMP_grdClubs\"]/tbody/tr/td[2]")[0].InnerText;
    }
    catch
    { }

    if (index != null)
    {
        var indexResult = Convert.ToDouble(index);
        return indexResult;
    }
    else
    {
        return null;
    }

Becomes this:

index = htmlDoc.DocumentNode.SelectNodes("//*[@id=\"ctl00_bodyMP_grdClubs\"]/tbody/tr/td[2]")[0].InnerText;
Return index != null ? Convert.ToDouble(index) : null;

You should also use SelectSingleNode instead of SelectNodes so that you return a single node instead of an array of nodes.

If you really don't want to the method to stop the whole process then you need to put something into the Catch that will let you know that something happened, logging would be preferable, but you could also put in a negative number for the index of that Golfer and then you would know which Golfer the Exception happened on and you would be able to report on that information as well...(I assumed that these "index" numbers are all positive or null.)

you could also change your

public async Task<ActionResult> UpdateHandicap()
{
  //Fetch all the golfers
  var results = db.Users.ToList();
  //Iterate through the golfers and update their index value based off their GHIN #. We store this
  //value in the database to make our handicap calculation
  foreach (Users user in results)
  {
      user.Index = await Calculations.GrabIndexValue(user.GHID);
      db.Entry(user).State = EntityState.Modified;
      db.SaveChanges();
  }
  return RedirectToAction("Index", "Users");
}

into a Linq statement(ish) and do what @t3chb0t said about saving the changes outside of the loop as well.

another thing that someone mentioned in chat was that you may not need to mess with the EntityState because EF usually handles that fairly well on its own. so your code would look more like this

public async Task<ActionResult> UpdateHandicap()
{
    var results = db.Users.ToList().ForEach(user => user.Index = await Calculations.GrabIndexValue(user.GHID));
    db.SaveChanges();
    return RedirectToAction("Index", "Users");
}