I think shirt will be great dude, it will be like a team or like rock band members (linkin park). Lastly, it is you who has to decide.
My three closest friends and I want some article to all match each other, what do you think we should go with? Shirt? Braclet, or anything else?
Matching shirts, or matching shirts with a favorite word from you all, that when you all stand together the word is completely spelled out.
Matching bff necklaces.
Shoes. Go shopping with your friends and buy white Converse. Come home and decorate each other's shoes with a message from each to each of you. Wear them when you hang out.
An arm band or a bracelet
Get two sets of matching ear rings, and all of you wear one in the same ear.