rustybroomhandle

Allows a Buddypress group administrator to customise their group with CSS via the “Group Custom CSS” tab under the group’s Admin page. Also supports preset themes.
NOTE: Even though this plugin can just be dropped in and used on your site, the css in the code is only really garaunteed to work with the default Buddypress theme, and it is assumed that you as a site builder will customise it to fit your site’s needs.

Buddypress Group CSS

Allows a Buddypress group administrator to customise their group with CSS via the "Group Custom CSS" tab under the group's Admin page.

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.

Buddypress Group CSS vs Groups 404 Redirect