Easiest way to get a notification when someone signs up?

Don't be lazy. it's just need small amount of code to automaticaly send an email everytime your user signup. You need faster? I write it in about 10 minutes, no need learning new thing, no need to pay something, just don't be lazy

You could text yourself!

const request = require('request');
request.post('https://textbelt.com/text', {
  form: {
    phone: '5555555555',
    message: 'New signup!',
    key: 'textbelt',
  },
})

I don't know how many new users you get but key=textbelt will give you a free text every day, or you can pay a few $ for more quota. (Disclaimer: I run https://textbelt.com)

Sending an email in nodejs is super easy (e.g. https://www.w3schools.com/nodejs/nodejs_email.asp ) so it's hard to imagine you could really find something simpler or faster.

Have your app call a Zapier webhook and then do the Zapier magick until your heart explodes.

(e.g. have it send you a text message or email)

I have done this, basically, I have a lambda that sends emails triggered by an SQS message. I use it for a lot of notifications and I've just added a snippet on sign up to send a notification to my email.

Alternatively, you may want to add those who sign up to MailChimp and assign them a tag. Use MailChimp API for that. This will be helpful for you in the future to kick off your email marketing, automated emails, follow-ups and such. Then in list settings set it to notify yourself when new subscribers added to list. https://mailchimp.com/help/change-subscribe-and-unsubscribe-notifications/