It's called SignUp Genius, and here's how it works:
- Build your SignUp: choose a theme and enter the dates/times and slots you need.
- Invite group members: via email; you can make your sign up public or private.
- People sign up online: accessible 24x7, and email notification is sent to you when someone signs up.
- Reminders are sent: customize when they're sent; members can receive via email or text.
I also noticed in the Nonprofit SignUps section they also provide tools to...
- plan event fundraisers
- manage donation lists
- plan service projects
For an example of what it looks like when people participate... I won't give you a link to my church's SignUp Genius form, but here's a screenshot of it (with last names photoshopped-out by me):