My
toolkit has a template Employee Manual as well as materials for starting and managing your own desk sharing program.
If that's not in your budget, though, I'd recommend trying to carve out some time to jot down your notes on your operations and anything someone might need to know as they go about their day. You're probably going to have to start writing this sort of thing down eventually anyway!
If you make that in a Google Doc or on something else collaborative, you can invite the volunteers (and even other members!) to help you update it as you go.
Hope this helps!