This code shows how to invite users to an Office 365 tenant as guest using Microsoft Graph API from an Azure Function

azFuncInvitation.js

module.exports = function (context, req) {
  context.log('JavaScript HTTP trigger function processed a request.');

  if (req.query.email && req.query.name) {
    const userEmail = req.query.email;
    const userDisplayName = req.query.name;

    getToken().then(token => {
      /* INVITE A USER TO YOUR TENANT */
      var options = {
        method: 'POST',
        url: 'https://graph.microsoft.com/v1.0/invitations',
        headers: {
          'Authorization': 'Bearer ' + token,
          'content-type': 'application/json'
        },
        body: JSON.stringify({
          "invitedUserDisplayName": userDisplayName,
          "invitedUserEmailAddress": userEmail,
          "inviteRedirectUrl": invitationRedirectUrl,
          "sendInvitationMessage": true,
          "invitedUserMessageInfo": {
            "customizedMessageBody": "This is a custom body message"
          }
        })
      };

      request(options, (error, response, body) => {         
        if (!error && response.statusCode == 201) {
          var result = JSON.parse(body);
          context.log(result);

          context.res = {
            status: 200,
            body: {
              id: result.id,
              inviteRedeemUrl: result.inviteRedeemUrl,
              status: result.status
            }
          };
        } else {
          context.res = {
            status: response.statusCode,
            body: result
          };
        }

        context.done();
      });
    });
  } else {
    context.res = {
      status: 400,
      body: "Please pass a name and email on the query string"
    };

    context.done();
  }
};

How can I use the getToken() function here?
Where I can import this function?