How can I add my pre-commit hook to an existing git repository

checking it out my colleagues have it instantly in place

Sept. 2015: That is not possible: a hook can be put in source control (you simply copy the script in your git repo), but it cannot be "automatically in place" (active) on clone/checkout: that would be way too dangerous, depending on what the hook is actually doing.
See also "Git remote/shared pre-commit hook"

You would still need to activate it, by adding a symlink (even on Windows) to the pre-commit hook present in the git repo.

Update Dec. 2016: the OP Calamity Jane mentions in the comments:

I solved it by now in symfony2 projects (and with others, it also should work) to have it as part of the composer.json.
So if a colleague is doing a composer install or composer update, it is automatically placed in the correct place.

"scripts": { "dev-install": [ "bash setup_phpcs.sh" ] }, 

So on dev, setup_phpcs.sh is automatically called and that copies the hook from a folder in the repository to the right place.
And since the hook is part of the repository it can be easily updated and distributed.

As noted by Hi-Angel in the comments:

I figured that out: Emacs has build-aux dir with hooks, and, upon running autogen.sh, all hooks are copied from there.

If your repository is an npm application, you can add this nice library as a dependency: Husky

File package.json

...
"devDependencies": {
    ...
    "husky": ">=4"
},
"husky": {
    "hooks": {
        "pre-commit": "npm test"
    }
}

pre-commit can be basically any command: if it ends with exit code 0, the commit passes, otherwise the commit is interrupted.