Every group is required to have a Seneschal and a Reeve. Also, every Barony is required to have an A&S Officer, a Marshal (called a Knight Marshal) and a Herald. Every Shire, Canton or College is required to have at least one of that set of three.
If your group doesn’t have an officer of a particular type and you discover you need one, obviously the ideal thing to do is to recruit one. If that’s not immediately possible, here’s what to do instead.
Marshal: if you want to run heavy fighting, find someone who is authorised as a heavy marshal or the insurance won’t cover you.
Arts & Sciences Officer: just do it! You don’t need an officer to be arty!
Herald: find someone who has the necessary skills, either locally (for voice heraldry) or online using the Facebook group.
Captain of Archers: if you want to run archery practice, you need someone who is an authorised target archery marshal or the insurance won’t cover you.
Rapier Marshal: if you want to run rapier practice or a tourney, find an authorised rapier marshal or the insurance won’t cover you.
Equestrian Marshal: if you want to run equestrian activities, find an authorised equestrian marshal or the insurance won’t cover you.
Keeper of the Lists: find someone who knows how to set up a tournament list, and keep careful notes to pass on to the Marshal.
Chronicler: anyone can publish things, but it won’t be an official SCA publication, so don’t present it as such.
Webwright: don’t present your website or Facebook group as official unless it is; otherwise, go for it!
Historian: just do it!
Hospitaller or Chatelaine: just do it! Everyone can and should welcome newcomers.
Chandler or Gold Key: just do it! Everyone can lend out stuff to people who need it.
Youth Officer: subject to child safety laws, just do it!
Chirurgeon: if you have first aid qualifications and there’s an emergency, don’t stop to consider bureaucracy. Save the patient, then give the Seneschal your notes.
Constable: find an authorised constable to do what must be done.