What sort of descriptions should I add?
It's really important that you go all out to add descriptions to every event, activity and task. Try to picture a brand new parent reading it. Are they made to feel welcome, or might they worry they are stepping on somebody's toes? What would they need to know about the task in order to muck in?
Say you need somebody to order some food for an event. Will you provide them with the shopping list, or will they be have to figure out what's needed and get it right? Probably the former, but if people are left guessing they usually decide not to take a risk. Take the time to fill out these details - assume zero knowledge.
Try to add some personality if you can -- some warmth -- rather than the "Peel potatoes" task having a description that says "Peel all the potatoes", try "No need to have professional experience of potato-peeling. We'll take anyone! We'll even make you a cuppa!" A friendly vibe goes a long way :)
The more welcoming and detailed you make it, the more likely that a newbie will dip their toe in. Anyway, writing it all down means next year the PTA team taking over has everything they need to simply copy and repeat the event with one-click! Result.