I'd appreciate it if Sb could check these sentences. I doubt if I know what they really mean.
"I agree with you."......(Me and you both have the same ideas.)
"I agree to you."......(We have different ideas, but I want to put an end to the quarrel or Sth.)
"I agree with that."......(I think it's morally acceptable.)
"I agree to that."......(I think it's a good suggestion.)
Agree to means say yes to somthing; you can't agree to people, you agree to things like plans, suggestions, and so on. Agree with means sharing opinions or having similar thoughts.