Create Unity Organization after signing up.
Go to Unity Editor and click the button in right top corner that says "Collab". It asks you to select the organization you want to share this project. After that you can check that Collaborate is enabled from services.
Click Collab button again (now it should have blue box with white arrow) and write the first commit.
This is similar to Git and there is no reason to use Unity's Collaborate feature unless you don't know how to use Git and you don't have interest or time to learn the basics.