var slackTeam = "YOUR_SLACK_TEAM_NAME ";
var token = 'YOUR_ADMIN_TEST_TOKEN';
var channels = 'C1234567890,G12345678';

// A test token will suffice.
// You can generate one at https://api.slack.com/docs/oauth-test-tokens
// Just make sure that the user issuing the test token is an admin.

var url = 'https://'+ slackTeam + '.slack.com/api/users.admin.invite';
fetch(url, { 
  method: 'POST',
  headers: {'Content-Type': 'application/x-www-form-urlencoded'},
  body: "token=" + token + "&email=" + input.email + "&channels=" + channels
})
.then(function(res) {
  return res.text();
})
.then(function(body) {
  var output = {rawHTML: body};
  callback(null, output);
})
.catch(callback);