Luis Rodrigues

If you have a private network where all sites are open to anyone with an account, but still want the ability to invite someone from outside and limit their access to a single site, then Network Restricted Members is for you.
This plugin was developed for our private <a href="http://p2theme.com" rel="nofollow ugc">P2</a> network, which is open to all company employees. However, we still wanted to be able to bring contractors and clients over without giving them access to everything on the network.
<h4>Usage</h4>
Network Restricted Members provides a user setting that allows multisite network administrators to restrict a user to the sites he or she is a member of.
<ol>
<li>As a super admin, navigate to the Users dashboard</li>
<li>Click ‘Edit’ on the user you wish to restrict</li>
<li>Check the option ‘Restrict User Access’</li>
<li>Save your changes</li>
</ol>
From this point on, administrators will need to add this user to their sites before he or she is able to see them.
If you wish to lift the restrictions on a user, repeat the steps above but uncheck the option box instead.
<h4>Other plugins</h4>
Network Restricted Members works best when combined with a network privacy plugin, such as one of the following:
<ul>
<li><a href="https://wordpress.org/plugins/more-privacy-options/" rel="ugc">More Privacy Options</a> by David Sader</li>
<li><a href="https://wordpress.org/plugins/network-privacy/" rel="ugc">Network Privacy</a> by Ron Rennick</li>
</ul>

Network Restricted Members

Restrict user access to selected sites on open multisite networks.

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.

Network Restricted Members vs Groups 404 Redirect