"I agree" means that you  agree with something that has just been said. Anna: "We seriously need to be doing more to stop the wildfires." David: "I agree." The statement "I agree with" is not a whole sentence. If you use this, you have to say what you agree with, or whom you agree with. Anna: "We seriously need to do more to stop the wildfires." David: "I agree with you. It's terrible." Anna: "That test was too hard." David: "I totally agree with that." When agreeing with a statement, we usually use "that." We also use "I agree" as part of a sentence: "I agree that..." Anna: That test was too hard." David: "I agree that it was hard, but I have taken harder tests." I hope this helps.

Thank you very much.

Hello, there is no difference in meaning, but we need to use the preposition WITH each time we say/write a complement for the verb agree. Example: I agree with you. In this case, YOU is the complement of agree, then with need to connect the verb and its complement using WITH.