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.

taras.sych

This plugin is as simple as you expected! 
And it support latest WordPress Gutenberg editor now!
Mark the content you want to hide as restricted with just a checkbox – and it’s done!
In case anonymous user will try to get access to restricted page or post – it will be redirected to WordPress Login page first. 
Or you can specify the exact URL (internal or external) to redirect anonymous users to.
After successful authorization, user is redirected back to the requested page.
Also, it hides restricted posts from Archive and Categories pages, RSS feed and from Latest Comments/Posts widgets!

Anonymous Restricted Content

This is simple enough to use and accomplishes the task. You can protect attachments, which was key for me. Support was slow.

Does what it needs to

Simple but yet effective plugin to hide selected posts and pages from anonymous users.

Network Restricted Members vs Anonymous Restricted Content