Co-option is when a new member is appointed to a board because of a paticular skill set or area of interest, usually on a short term basis. It is usually done during the year and not at the AGM. It is not permitted by all governing documents so you should look carefully at your organisation's governing document before considering co-opting someone on to your board. Your constituion may also provide for different voting rights etc for the co-opted person and this needs to be made clear.
An example of a co-option form which can be used in the recruitment process.
Always ensure that example documents are tailored to suit your organisation.