Creating a Share Function

I’m making a to-do app that’s similar to the app on Apple devices, but I want to create a share function where you can share the to-do list with other people and they can add to the list, edit the list, etc. How would I go about creating that function?

This could be done with a cloud database, like Firebase. People would be able to log into it, add users and add each other to a certain list.

Or! You can even use CloudKit

Thanks for your input!
I followed Chris’ tutorial on Firebase authentication and I have the sign up/login functions. However, I don’t quite know how to allow the users to add each other to a certain list. Any ideas?