User invite email not delivered - related to subdomain?

This was a long time ago, but I think I remember my solution being reinstalling Wordpress as a regular site instead of a multisite (it turned out that I didn't need it to be a multisite). Obviously that's doesn't get to the actual root of the problem, but just posting here in case it's useful to someone else.

(I know this is a late answer, so the original poster has probably moved on from this. But I am posting an answer to this in hopes that it will help later users solve their issue, should they happen along to this question.)

The first step is understanding how email works in WordPress. WP will use wp_mail() to send messages, and this is essentially a wrapper for PHP's mail().

The default "from" address will be an email address configured based on your site's setup. And unless specifically configured to do otherwise, the message will be sent using the web server's emailer.

My host is also not handling the email for this domain, since I have my MX records at the registrar pointed to G-Suite.

That's not actually a true statement. Where emails from WP are sent from/through has nothing to do with your domain's MX records. These are two totally separate things.

Now, that being said, you can configure WP to utilize your external email account (in this case, G-Suite). But that's not automatic.

There are two ways to do it. One approach is to use an SMTP plugin. There are many of these available, and a search of the wordpress.org plugin repository will yield several good possibilities.

There are simple approaches to do it without a plugin as well. All you need to do is define your email account information so that the mailer can connect to it.

So to come back around to the initial issue - "the user is not receiving the email with the confirmation link" - it is possible that your emails are not being sent by your host.

(Note that when/if they are sent through an SMTP account, you generally will be able to look in your sent folder to confirm an email was sent. With your web host's email server, which undoubtedly is what is being used in this situation, you may or may not have access to the sending queue to review whether an email was sent.)

There could be a number of reasons why an email was not sent or received. But configuring your setup to use your SMTP account will negate most of those possibilities and make it easier to solve other possibilities.