Right let us clarify that you can’t simply test users using a membership table that you have added users to and have access to their email address.
Plus, as Admin of your website you will not receive notifications of your own blog posts or forum posts etc, for you to receive them yourself, you will need to signup as another site member with another email address that is not the same one as you use for Wix.
I have done that myself so that I can test anything before I send it by simply sending it back to my testing member as Wix classes it as another site member even though it comes back to myself.
As for your members, all your members need to physically go through the signup process to your website to become site members, either through Wix own window or custom window or your own custom signup lightbox.
You can’t simply add members yourself, they all have to go through the Wix Users .register() function which then adds the users details to the Wix CRM and then you can see these details in your Contacts List.

If you go to your Contact List and use the Filter by dropdown you will actually be able to see who is subscribed to your website or not - note that it will be under the label of Subscribed .

This Contact List is through Wix CRM so it displays all your Contacts and this includes contacts who are not members.
https://www.wix.com/corvid/reference/wix-crm.html
https://www.wix.com/corvid/reference/wix-crm-backend.html
Once a user becomes a site member then you use Wix Users to get their info and do things with their details etc.
https://www.wix.com/corvid/reference/wix-users.html
https://www.wix.com/corvid/reference/wix-users-backend.html
What’s the difference between wix-crm and wix-users?
The CRM API contains functionality for working with your site’s contacts.
The Users API contains functionality for working with users who are logged-in members.
Note that all members are also contacts, but contacts are not necessarily members.
Note
The APIs in wix-users are only partially functional when previewing your site. View a published version of your site to see their complete functionality.
They also need to do this so Wix can give them a unique Owner ID so it knows who the site member author is when needed to through the permissions, see the system field info here for more about it.
https://support.wix.com/en/article/about-your-database-collection-fields#system-fields
When a user is a site member, then they will also be listed in your Site Members tab too.

If you click on any contact or member from either list, it will bring up their details and you will be able to see on there what status they are and if they are subscribed to your website or not.

Once your members have all this setup and they are also members of your site’s community, then they go to the Wix Blog and either join the community there if they are not, or they can change their own settings as to whether or not they are receiving notifications.
https://support.wix.com/en/article/becoming-a-new-blog-member-and-joining-the-blog-community
Each member must do this themselves after they have logged in themselves using the Wix Users .login() function, you can’t do it just through having an email address setup for them.
https://support.wix.com/en/article/sending-email-notifications-about-new-posts-in-the-new-wix-blog
Just because you have the toggle setup to send email notifications, if the members themselves are not members of your site’s community or have not got their own settings turned on to receive those email notifications, then they will not get them.