Just start adding tasks. Only sign up if you want to save more than one list.
Copy/paste the URL to share a list. No permissions. No formal invites. It's that simple.
Browser based so your lists are always synced and you can access them from anywhere.
What I like about flask is the simplicity, ease of use, and lack of any barriers to get started.
I have been using it for a variety of things including to-do lists, shopping lists, goals, items-to-pack and wishlists.
The fact it is available from anywhere makes it extremely versatile.