The verb zustimmen is used with a dative, whether you're agreeing to a person or to a statement.
(a) What do we do if we want to say "I agree with that"? Would
Ich stimme das zu
(b) What about "I agree that ..."? Would it be
Ich stimme zu, dass ...
Or should we add some da-compound, like
Ich stimme damit zu, dass ...