'I agree with you' is correct. Since 'agree' is a verb (not an adjective) you would need to use the continuous tense to include the verb 'to be' (am). For example: I am agreeing with you - however, this expression is not as common and might imply that it is an usual action. For instance, I am agreeing with you now, but usually our opinions differ a lot.

I agree with you is correct