You agree with someone (person) You agree to do something (thing)

'Agree with..' means that you share an opinion. It's usually followed by a noun. You can agree with a person: 'I agree with you', or with a thing: 'I agree with this statement/policy/idea'. 'Agree to' is followed by a verb. For example, if you agree to do some work, you're saying 'Yes, OK, I'll do it.'

This is an interesting question. Many teachers of English are still teaching: ~ agree with someone ~ agree to something (see http://dictionary.cambridge.org/dictionary/english/agree-to-sth"