The Simultaneous Membership Program allows you to attend Army ROTC and serve in the U.S. Army Reserve

Simultaneous Membership Program (SMP)

The Simultaneous Membership Program allows you to attend Army ROTC and serve in the U.S. Army Reserve
or Army National Guard at the same time. It gives you an opportunity for additional training and experience. Cadets serve as Officer trainees in the Reserve or National Guard while completing college. You can earn Reserve/Guard pay and benefits in addition to your Army ROTC allowances.

http://www.goarmy.com/rotc/enroll/enlisted.html