Another way of saying I agree is to say Yup.
Or You're exactly right or that is correct.
Other ways to say I agree is to say "Exactly" or Absolutely.
To agree with someone is to consent to do something that has been suggested by another person or have the same opinion about something.
You can also nod your head to say you agree with someone or simply just say I agree with you.
Some examples of agreeing with someone are.
That's right/You're right/I know: used when agreeing with someone.
Exactly/Absolutely/I couldn't agree more: used for saying that you completely agree with someone.
You can say that again/You're telling me: a more informal way of saying that you completely agree with someone: