The Request Access plugin is intended for WordPress website owners who want to allow people to sign up for their website in a controlled way.

Noted That:

To install correctly this request-access.zip.
Fisrt Download the request-access.zip to your computer
Extract/Open request-access.zip to Your Computer.
Then, Find readme.txt file inside request-access.zip and Open readme.txt.
Now, Read the Requirements of this plugin. Which Wordpress Version and PHP Version are required to run this Plugin in Your Wordpress Site.
Then, Follow the the Tips Below.

Start the Tips:

1. First Download "request-access.zip" Plugin to your Local Computer. (Click Download)

2. Then, Login to your "yourdomain.com/wp-admin" Dashboard.

3. Then, Click on "Plugins" + "Add New" from left sidemenu of Dashboard.

4. Now, Click on "Upload Plugin" button.

5. Now, Browse "request-access.zip" Downloaded plugin from your computer, Where you downloaded request-access.zip According to Step – 1 Above then, click on "Install Now"

6. Now, Click on "Active Plugin"

7. Then, See left sidemenu. "Request Access" folder is added on left sidemenu. Now, Click on "Request Access" folder.

Noted that: If you do not see "Request Access" folder on left sidemenu then, see at left sidemenu "Settings" or "Tools".

8. Now you configure yourself oR Watch video tutorial below about Request Access Configurtions and Settings or How to work "Request Access" in your WordPress site.

After Activated Plugin According to Step-6 then,

Go to "Plugins" + "Installed Plugin" from Wordpress Admin Panel Leftside Menu.
or Direct go to: https://yourdomain/wp-admin/plugins.php
Then, Find "Request Access" Activated Plugin from Plugin List.
Then, Click on "Settings" from Plugin that is Request Access
Now, Edit/Add/Config the setting and Click on "Save Changes" button,

Guide
Install the plugin by uploading it to your wp-content/plugins directory and activating it. There will now be a new section available in your admin area, visible only to administrators. The Request Access plugin creates three new pages: Request Access – This is the main functional area of the plugin where you can view pending accounts and approve or deny them. It is visible to any account types selected in the settings. Log – This is a log of all the admin actions. It is only visible to adminstrators Options – This lets you control the options and settings of the plugin The plugin requires a number of pages to be created, each of which serve a different purpose in the public flow and subsequent approval/denial flows. All of these pages are public by default; you don’t need to manually set them as public (though nothing bad will happen if you do): Sign Up page – This is the page that contains the Sign Up Form. Add the form itself to the page with the shortcode [signup], which allows you to control the text/content that appears before and after the form fields. Log In page – This is the page that contains the Log In Form. Add the form itself to the page with the shortcode [login], which allows you to control the text/content that appears before and after the form fields. Thanks page – This is the page the user is redirected to after they complete the signup form. You control all of the content on this page. Pending page – This is the page the user sees if s/he attempts to log in to the site while their account is still pending. You control all of the content on this page. Denied page – This is the page the user sees if s/he attempts to log in after their account has been denied. You control all of the content on this page. The plugin has a few more options that need to be configured before it can be fully used: Notification recipients – This is a comma separated list of email addresses that should be notified whenever a new user signs up and is pending Approved Email Subject / Body – This is the email subject/body that is sent to a new user if his/her account has been approved Denied Email Subject / Body – This is the email subject/body that is sent to users who’s account has been denied. Roles allowed to approve/deny pending requests – This is a list of roles on your website that should be allowed admin access to approve/deny pending users. Check each role that should have access.

Guide

Install the plugin by uploading it to your wp-content/plugins directory and activating it. There will now be a new section available in your admin area, visible only to administrators. The Request Access plugin creates three new pages:

Request Access – This is the main functional area of the plugin where you can view pending accounts and approve or deny them. It is visible to any account types selected in the settings.
Log – This is a log of all the admin actions. It is only visible to adminstrators
Options – This lets you control the options and settings of the plugin

The plugin requires a number of pages to be created, each of which serve a different purpose in the public flow and subsequent approval/denial flows. All of these pages are public by default; you don’t need to manually set them as public (though nothing bad will happen if you do):

Sign Up page – This is the page that contains the Sign Up Form. Add the form itself to the page with the shortcode [signup], which allows you to control the text/content that appears before and after the form fields.
Log In page – This is the page that contains the Log In Form. Add the form itself to the page with the shortcode [login], which allows you to control the text/content that appears before and after the form fields.
Thanks page – This is the page the user is redirected to after they complete the signup form. You control all of the content on this page.
Pending page – This is the page the user sees if s/he attempts to log in to the site while their account is still pending. You control all of the content on this page.
Denied page – This is the page the user sees if s/he attempts to log in after their account has been denied. You control all of the content on this page.

The plugin has a few more options that need to be configured before it can be fully used:

Notification recipients – This is a comma separated list of email addresses that should be notified whenever a new user signs up and is pending
Approved Email Subject / Body – This is the email subject/body that is sent to a new user if his/her account has been approved
Denied Email Subject / Body – This is the email subject/body that is sent to users who’s account has been denied.
Roles allowed to approve/deny pending requests – This is a list of roles on your website that should be allowed admin access to approve/deny pending users. Check each role that should have access.

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Reffered: https://wordpress.org/