The Standing Committee is a 14-member group (15, if the Archbishop of Canterbury is present, as he is an ex officio member, as well as being its President). Seven of its members are elected by the members of the ACC, and five are members of the Primates’ Standing Committee. The other two members are the Chair and Vice-Chair of the ACC, elected by the members in plenary session. Their function is together to assist the Churches of the Anglican Communion in advancing the work of their mission worldwide.
There is a Q and A about the Standing Committee here which has further information.