As far as the difference in meaning between: Agree & Agree With… There is none. For, in order to “Agree” — it is necessary to do it “With” another person, idea, opinion, philosophy, etc.. (All of which would come from another person.) .
Therefore — the only difference between the Verb: “Agree”, and the Phrasal-Verb: “Agree With” — is in how these two are used.
When we use “Agree With” — there needs to be a Noun or Pronoun (acting as a DirectObject) directly after it in the sentence. This is usually a person, but can also be an: Idea, Opinion, Philosophy, etc.. .
To use the word“Agree” without the DirectObject, is usually as a statement in response to another, or in answer to a question — and the word“Agree” will almost always be the last word in the sentence… (but it can also be followed by an adverb, an adverb-phrase, or a phrasal-adverb.)
“I agree with you — that we need to make learning English much more simple.”
“He said that we needed to make English more simple to learn. I told him that, I agree. And then we slapped high-five and did a little dance before running off into the sunset.”
“What do you think about traditional English lessons being boring and stupid?”… “I agree, whole-heartedly. That’s why I use GiveMeSomeEnglish!!!” 😎
The statement “I agree.” as a complete two-word simple-sentence — which stands on its own — is also classified as an Interjection. The person does not need to say “I agree with you.” because the person being spoken-to already comprehends that he or she is the one being spoken-to.