How do I share Keynote presentations and perform collaborative editing on iCloud?
I save the document on my Mac or my iPad and then quit keynote.

Then I open https://www.icloud.com/#keynote in Safari 9 on a computer (most other newishly updated browsers work too)

If you don't see the "coaching tips" - click the white ? in the white circle in the top right next to your name.


Double click your keynote you want to share. (It might take a while to load the first presentation - so be patient while it loads)
Now look in the "top bar" of the web browser window to the left of the ? are in order: your name, a wrench, and the share icon. 
Click the box with the up arrow
Share away. This generates a URL you can copy to SMS/message/email/post as well as prompts you if you want read only or write access and lets you optionally set a password.


You can also share from iOS keynote and Mac keynote, but the steps there depend more on the version you are running.