It will be fine; volunteering is a very low-stress situation. So long as you're johnny (or jane) on the spot, they'll love you.
Use it as an opportunity to learn. You'll have direct access to PTs so ask them any questions about physical therapy, the day-to-day life of the clinic, any preparation they did prior to PT school, etc. Get to know them as they could be potential references. Be friendly and courteous to patients. This is an opportunity to develop your "bedside manner." And treat the PTAs, techs and transport staff like you would the PTs. Good luck and have fun!