Simply post here with your gamertag.
I've actually just updated the OP with more information, take a look at the section headed "That's all great but how do I join this crazy thing?" for more details. You'll want to get your TA game collection up to date first.
I'm happy to roll you a list for April, but as you pointed out there's only a few days left so that may (hah, "May") not be ideal. Up to you though, I don't mind.