script.js

function invite() {
    const TOKEN = ""; //add your token here, generate from https://api.slack.com/custom-integrations/legacy-tokens
    const EMAIL_LIST = [
       
    ];

    //add all IDs of the channels you will want the users to be added automatically to
    //you can get a list of your channels at https://api.slack.com/methods/conversations.list
    const CHANNELS = "CC3BY78PP,CC3BFNZMK,CC33EUZ8T";

    //loop through the array of emails and send the invite
    EMAIL_LIST.forEach(email => {
        sendInvite(email);
    });

    function sendInvite(email) {
        const INVITATION_URL = `https://slack.com/api/users.admin.invite?token=${TOKEN}&email=${email}&channels=${CHANNELS}`;

        fetch(INVITATION_URL)
            .then(response => response.json())
            .then(function(response) {
                console.log(`${email} invitation status:`, response);
            })
            .catch(function(error) {
                console.log(`${email} invitation status:`, response);
            });
    }
}