There are several synonyms for the phrase "I agree" which can be used depending upon the person and the context. One of the common phrases used to show agreement is "I concur". Other simple phrases for agreement include "I support", "I endorse" and "I approve". If you want to emphasize your agreement, you could say "I fully concur with you" or "I wholeheartedly agree". Sometimes, you may choose to express agreement by saying "You're absolutely right" or "I couldn't agree more". Whatever the phrase you choose, your intention is to show that you share the same opinion as the other person.