As Blizzard makes it clear here you can catch an advertisement on your Xbox that will let you get a special shirt for your Live character.
The shirts look good.
Signed RedFlare up with Live just so we could get the female versions too.
Now to be clear, you have to be in the standard Xbox configuration. I had to mess with my settings pretty heavily because I’d been part of a Xbox Live family at the time so I couldn’t get my shirt.
It was worth it to leave, at least temporarily, to get the shirt. Haven’t worried about rejoining the group quite yet. Your access to the shirt will probably be easier than mine was though.