For me being materialistic means to have many possessions and to want to only get more of them. Being in a group doesn't mean you are materialistic though.
It means you always want to get something. And you are concerned only about getting it and using money. It is important for such a person to socialize only for their own selfish reasons. Perhaps they want to impress their mates as well by doing so. So we can say that they want to brag or show-off.
Maybe they want to follow a fashion and because of that buy many kinds of fashionable products.
Music can also influence their behaviour. Some popular music's lyrics can encourage being materialistic.
Such people are also usually into labels and brands. Lastly, they most likely would not be happy with secondhand clothing. They would prefer brand new clothing.