How would I play this
 

I'm still relatively new to playing guitar, but I really want to learn this song, but I can't even pull of the first shape he does. I'm using this tutorial video. The shape is 4x644x with the thumb over the top, and first finger barring strings 2, 3 and 4 on the 4th fret, ring finger on the 6th fret of the A string. I keep moving between muting the string below the one my ring finger is fretting and not barring properly because I've had to move my hand to compensate.

I'm wondering if this is something that most guitarists can play easily, that can just be learned with enough practice, or if it's something only people with longer fingers can do?

You say you are relatively new to playing guitar. This isn't really beginners stuff. If you can recognise the chord names from the instructional video a preliminary step could be to learn to strum the basic chord sequence first and try to put in the hammer-ons later. It's not completely impossible but it will not be easy.

It's a G#m and you should just play it with another fingering. The fingering you've posted is skipping the D# on the fifth string because it's repeated again on the second string. So, play a regular G#m barre chord, place your ring finger on the fourth string and not your pinky as would be standard, and then "cheat" your ring finger over a little so it mutes the fifth string.

Like so:

http://i40.tinypic.com/8wipao.jpg

(my middle and pinky fingers are tucked behind the neck so you can see the fretboard...you don't have to do that)

Also, if you're relatively new, I wouldn't attempt this yet. :)

As stanron says, this is not a beginner tune, at least not as John Mayer plays it!
The chords themselves could be played by anyone who's used to playing barre chords; and the shape in this song is a little easier because the index only needs to hold 2 strings: 3 and 2, not 4; it can fret 4th as well, but it's not necessary. The thumb is fretting the 6th, which is how John Mayer played it, but it could be done with index.
But it's more the basis of a riff than a rhythm chord as such.

The tab for that opening phrase is this:

----------------------------
-----4-----------------------
-----4h6p4-----------------------
-------------6---------------
----------------------------
-4---------------------------

- so you're not actually playing all the strings at the same time.
I see the main difficulties as (a) getting that hammer-on/pull-off quick and clean enough, and (b) muting unwanted strings.
Both could be serious challenges for anyone "relatively new to playing guitar", and I'd suggest not attempting this riff.

Even the simple barre chords for strumming the rhythm may be too much of a challenge - but it's worth bearing in mind that you don't need all 6 strings to sound. Eg, this is a plain G#m chord (full barre version):

-4- root
-4- 5th
-4- 3rd
-6- root
-6- 5th
-4- root

As long as you're getting the 6th string root and 2 or 3 of the other notes, the chord will work well enough, provided no open strings sound.
IOW, if your fingering mutes some strings instead of fretting them, that may be no big deal. (If you have at least one example each of root 3rd and 5th, you have a complete G#m chord ;).)
You can fret 6th with your thumb if you want, or go for the traditional full index barre.
What matters more is getting the rhythm right, establishing that groove.

Ok first of all, thanks for all the detailed replies, they're much appreciated! I know this song is probably too difficult for me at this stage but I have a real problem finding songs at the right difficulty level. I think I'll try to barre it and see how it goes. The problem is I still find changing between open chords and barre chords tricky. Guess that's just practice though. I hate the feeling of not playing a chord properly and having strings muted though. My inner perfectionist isn't comfortable with the compromise!

Right, but don't forget rhythm and timing is more important than getting all the right notes. Focus your sense of perfection on the groove, at least as much as on the fretting of your chords.
Of course you need to avoid wrong notes! - but not all the right notes need to be there all the time. Keeping the beat is the fundamental essential; the right notes get built on that (one by one if necessary).

This is a really interesting post. I know this is off topic, but we hear from so many teachers that we ought to practice slowly and get all the notes correct before we take things up to performance speed. Yet other people offer the advice above, which is to concentrate on rhythm first. These are two entirely different recommendations, yet each is offered with the assurance that it is absolute truth.

There are a zillion technically excellent guitarists on this forum, but the one thing I repeatedly hear lacking in modern acoustic playing is any sense of a rhythmic groove. Stefan Grossman likes to remind students that all this stuff used to be dance music. That idea seems to have gotten lost in the search for technical wizardry.

You can practice rhythmic aspects separately from any given song. Groove involves tempo + rhythm + playing the right notes with the right volume (accenting). Good material (hooks and the like) doesn't hurt.:)