jms,
I have been planning a similar trip and have been looking for options. Aside from the shuttle you mentioned, there is a YARTS bus that leaves once a day at 5:00pm from the Visitors Center in valley floor. Its a $8 fare one way and arrives around 6:45pm at TM.
http://yarts.com/
I have also heard many people hitch hike, but a for group your size...maybe not.
Please post any other options you find.
Thanks,
Gabe