To express the concept of "I agree" you can use the signs THINK and SAME.
Point to your forehead and then use the sign "SAME/similar" in a directional
manner between you and the person with whom you agree.
Here's an example of "I think the same as you."
Also see:
AGREE
You can learn
American Sign Language (ASL) online at American Sign Language University ™
ASL resources by Lifeprint.com © Dr. William Vicars