It's pretty simple, you just append your key to the URL as a parameter.
For example, looking for users 1, 5, and 15:

http://api.stackexchange.com/2.2/users/1;5;15?site=stackoverflow&key=YOUR_KEY_HERE

HTTP.call (
    "GET", 
    urlString, 
    {params:{site:"stackoverflow", key:"YOUR_KEY_HERE"}}, 
    function (error,result) {
        console.log (result.data);
    }
);

It's pretty simple, you just append your key to the URL as a parameter.
For example, looking for users 1 and 5:

http://api.stackexchange.com/2.2/users/1;5?site=stackoverflow&key=YOUR_KEY_HERE

HTTP.call (
    "GET", 
    urlString, 
    {params:{site:"stackoverflow", key:"YOUR_KEY_HERE"}}, 
    function (error,result) {
        console.log (result.data);
    }
);