Sign Up for your community gardens Admin account, then go to Admin Profile in your Settings and invite anyone you want to join. This account must be separate from your personal garden account (if you have one).
Now everyone in your group can see and collaborate on the same garden. Any of your group can put their name next to a To Do in the Weekly List, letting everyone know that they have the To Do handled.
Once the To Do is completed, it's checked off as "Done" and is automatically included in the Community Journal.