itthinx

This plugin redirects 404’s caused by hits on pages that are protected by <a href="https://wordpress.org/plugins/groups/" rel="ugc">Groups</a>.
The redirection settings can be adjusted in Settings > Groups 404 :
Visits to protected pages can be redirected to:
<ul>
<li>a specific post, or</li>
<li>the WordPress login</li>
</ul>
The redirection can also be applied for restricted categories, tags and other taxonomy terms if <a href="https://www.itthinx.com/shop/groups-restrict-categories/" rel="nofollow ugc">Groups Restrict Categories</a> is used.
The redirect status code can be selected among:
<ul>
<li>Moved Permanently (301)</li>
<li>Found (302)</li>
<li>See other (303)</li>
<li>Temporary Redirect (307)</li>
</ul>

Groups 404 Redirect

I know this plugin hasn't been updated in a couple years, but IMHO Groups and its related plugins (like this one) are a hidden gem for WordPress sites. So many people jump to complicated membership plugins when they could do everything they need with Groups. This 404 redirect add-on works like a charm. I'm using it on the latest version of WP 5.x

Perfect.

Redirect 404's when a visitor tries to access a page protected by Groups.

The plugin protects a site so that only members can access its pages.
Suitable for site lockdowns, private sites and sites that are under construction.
It allows to redirect visitors to a particular page or to the WordPress login.
Members who are logged in have access to the site. 
Members must belong to the site’s Registered group, or to a particular group as determined specifically for the site.
The plugin is an extension to <a href="https://wordpress.org/plugins/groups/" rel="ugc">Groups</a> which is required.
The redirection settings can be adjusted in Settings > Groups Blog Protect :
Redirection options can be set for the blog – or for each blog in a multisite setup:
<ul>
<li>no redirection</li>
<li>redirect to a specific post, note that with this option the blog is shown to the visitor but only that page can be visited</li>
<li>redirect to the WordPress login</li>
</ul>
The redirect status code can be selected among:
<ul>
<li>Moved Permanently (301)</li>
<li>Found (302)</li>
<li>See other (303)</li>
<li>Temporary Redirect (307)</li>
</ul>
Site administrators can determine which group is used to protect the site:
<ul>
<li>By default, users must belong to the Registered group to be able to access the site, so any registered user who is logged in will have access.</li>
<li>The constant <code>GROUPS_BLOG_PROTECT_GROUP</code> can be set in <code>wp-config.php</code> to indicate the name of the group to which users must belong to be able to access the site.</li>
<li>For multisites, the <code>GROUPS_BLOG_PROTECT_GROUP_n</code> constant can be used to indicate the name of the group required to access a particular site, where <code>n</code> is the blog ID of the site.</li>
</ul>

Groups Blog Protect

Protect access to blogs via group memberships powered by Groups.

Groups 404 Redirect vs Groups Blog Protect