In accepting the position of chairperson, Marcia was agreeing to attend all the meetings.
In accepting the position of chairperson, Marcia was agreeing with attending all the meetings.
In accepting the position of chairperson, Marcia was agreeing to attend all the meetings.
In accepting the position of chairperson, Marcia was agreeing with attending all the meetings.
To "agree to" something is to accept a duty with conscious intent. To "agree with" something is merely to think it is a good idea, but it implies no personal responsibility. Marcia is accepting personal responsibility, so the "agree to" idiom is correct.