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."
You can learn
American Sign Language (ASL) online at American Sign Language University ™
ASL resources by Lifeprint.com © Dr. William Vicars