How do you install ssh-copy-id on a Mac?

You can install it using Homebrew:

brew install ssh-copy-id

If you don't want to use Homebrew, you can use this Mac port:

curl -L https://raw.githubusercontent.com/beautifulcode/ssh-copy-id-for-OSX/master/install.sh | sh 

MacPorts version: sudo port install openssh +ssh-copy-id