The introduction of the song follows the same chord progression as the chorus:
Am,B,Am,Bb,Am,F,Am,F7,Am,E7,Am
(refer to lyrics to aid in understanding the timing)

Chorus
          Am
When the crypt goes creak,
           B
And the tombstones quake.
Am                     Bb
Spooks come out for a swinging wake.
Am           F
Happy haunts materialize,
Am           F7
And begin to vocalize.
Am                        E7          Am
Grim grinning ghosts come out to socialize.


Now don't close your eyes,
And don't try to hide.
Or a silly spook may sit by your side.
Shrouded in a daft disguise,
They pretend to terrorize.
Grim grinning ghosts come out to socialize.
---------------------------------------------
The song then modulates to B-flat minor for the verses. The modulation to B-flat minor 
can be achieved by
placing a capo on the first fret, and playing the chords as before. Or, if you prefer, 
you may also
use the barre method.
---------------------------------------------
First Verse

As the moon climbs high o'er the dead oak tree,
Spooks arrive for the midnight spree.
Creepy creeps with eerie eyes,
Start to shriek and harmonize.
Grim grinning ghosts come out to socialize.

Second Verse

When you hear the knell of a requiem bell,
Weird glows gleam where spirits dwell.
Restless bones etherialize,
Rise as spooks of every size.

Third Verse
Note: The song does not modulate to B-flat minor for this verse, but rather remains in A 
minor, so it is played
like the chorus

If you would like to join our jamboree,
There's a simple rule that's compulsory.
Mortals pay a token fee.
Rest in peace, the haunting's free.
So hurry back, we would like your company.


Chords and chord progression retrieved from wikipedia, at 
http://en.wikipedia.org/wiki/Grim_Grinning_Ghosts
The tab itself was composed by me.
Please do not be afraid to comment and leave any suggestions/corrections.