Occasionally, a Committee lacks some expertise which might have been contributed by a particular member had she been available for election and been elected, so most Rules empower a Committee to co-opt, if they wish, a specified small number of extra members for specified reasons. A co-opted member is not elected by the AGM but chosen by the Committee at any of its meetings and serves only until the next AGM, unless again co- opted.
The reason she never attends the first Committee of the year is simply that she could not have been constitutionally co-opted before it. She is almost always invited to all the other meetings if only because that is most convenient, but she might be asked to attend only when her expertise is likely to be required. She is not deemed an officer, does not count toward a quorum, does not vote on Committee, even on her own subject and though she may always speak on her own subject, on others may do so only with permission though, as she is an honoured member, normally it is given.
From Page 100 of The Committee Book by Audrie Stratford