Hire a lawyer. There is no guilty by association. You may be a party to a theft - meaning that you aid and abetted in some manner. You want to keep this off your record or keep your punishment at a minimum as theft is a crime of moral turpitude.
Sounds like the State is proceed on the case under 7.01 of the penal code - law of the parties and claiming you were a party to the offense. If the can prove this, then it would mean you are just as guilty as the other person charged. So, you should hire an attorney to defend you situation right away. If you have a Juvenile case in Harris County, I recommend Kathryn Kelly Graul - Former prosecutor and very good.
I am not sure what you consider to be "guilt by association"? If it is the "law of the parties" where you were an active participant, then I agree with the other answers posted. If you were just present and, perhaps, did not know about the theft before or during the theft, there may be room to argue. Recently, I had this type of situation. In the end, the active participant received a Pre-Trial Diversion and the charges agaainst the unknowing party were dissmissed. I also note that you may be a juvenile. If so, you still need an attorney to help evaluate your case.