I am about to be a junior in college. Is it okay to join the National Guard without joining ROTC?

Yes. The National Guard and ROTC are two diferent things. I don't understand that much, but I think that ROTC is only a program that trains qualified members to be officers in the army. On the other hand, the National Guard is part of the army reserves forces.

If you join the National Guard, you will be an ordinary enlisted part-time soldier, and after basic training and AIT, you will drill for one weekend every month and an additional two weeks every year. You will also need to be ready at all times in case of an emergency when the govorner of your state or the president calls your unit to active duty.

You can find much more information on the National Guard and ROTC websites.