A person on duty is ex officio?

This means, roughly, "by way of office". This term is often used by people following Robert's Rules of Order. A person may be a member of a committee because he was voted into the committee by the membership. Other committee members may be members because of other positions or offices that they hold in the organization. As an example, the organization's elected treasurer may be, ex officio, a member of the Finance Committee, and this is made clear in the organization's bylaws, or other documents establishing the organization's authority. In this example, the treasurer doesn't have to be nominated as a member of the Finance Committee, and there is no vote on this. As soon as one is elected treasurer, she/he is "ex officio" a member of the Finance Committee. Or perhaps the Mayor is ex officio the member of some committee. This membership is not "honorary", as is sometimes mistakenly assumed. It is a full membership with the rights and privileges that go along with membership, unless limited by some other actions of the organization.