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 repo is an npm app, you can add this nice lib as dependency: husky

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, commit passes, otherwise commit is interrupted.

fyi information in the year 2019 there is also this option:

require this package: "brainmaestro/composer-git-hooks"

and add the following lines to your composer.yaml

"extra": {    "hooks": {        "commit-msg": [            "regex=\"^([A-Z]{2,4}-[0-9]{1,4}|(no-ticket|NO-TICKET)):[\\s]*.{10,}\"",            "file=`cat $1`",            "if ! [[ $file =~ $regex ]]; then",            "  echo \"ERROR - Commit message is wrong or too short. E.g. XXX-33: Description or no-ticket : Description\"",            "  exit 1",            "fi"        ],        "pre-commit": [            "git status --porcelain | grep -e '^ [AM]\\(.*\\).php$' | cut -c 3- | while read line; do",            "ROOT=`php -r \"echo __DIR__;\"`",            "bin/php-cs-fixer fix -nq --config=$ROOT/.php_cs \"$line\";",            "bin/phpcbf --standard=PSR2 --encoding=utf-8 -n -p \"$line\";",            "git add \"$line\";",            "done",            "echo committing on branch $(git rev-parse --abbrev-ref HEAD)"        ]    }}

This is the example, which works for me. What it basically does is this:

Everytime you run "composer install" or "composer update" the hooks in .git/hooks are checked. If the hooks are already in place nothing happens. If they are missing, then the lines from above are parsed into the the hooks with a shebang at the beginning. They are then executed each time somebody is triggering a hook.

If you don't have big scripts that is IMO the better solution than copying scripts around.

Note: if you change the lines in the composer.json for the hooks you have to delete the respective hook first before you run "composer install" or nothing will change.