Decide which tasks need many hands. For example, planting, clearing, and digging are tasks that could use many unskilled volunteers (with a little training!). Focus volunteer days on those tasks.
Preparation tasks such as the placement and layout of elements require fewer, more knowledgeable volunteers (small group of 3-6 people). Do this work a day or two before larger volunteer days.
One week before any digging, visit https://www.georgia811.com/ to make sure there are no buried utilities where you are going to dig.
From their website:
By law, everyone – including homeowners – must contact Georgia 811 by visiting my.georgia811.com or by dialing 811 or 1-800-282-7411 at least 48 hours before beginning any mechanized digging on your property. If you are unsure as to whether you should notify Georgia 811 before you break ground, remember, even if your digging project is small, it’s always best to call!
Source materials and tools well-beforehand. You can request volunteers to bring their own tools, but you can also borrow tools from the Atlanta Tool Bank (https://www.atlantatoolbank.org/).