An overview of Co-option.
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. If the constitution does not make this clear then the committee should agree and minute the voting rights, length of term, etc for the co-opted person.
If the person is a full member of your committee and your organisation is a registered charity you will need to update the details for the Charity Commission.