Markus Echterhoff

If you have a multisite setup, you normally cannot have the same bbPress forums across all sites of the network. Now you can.
Install this plugin and activate it on the main site and for each subsite that should use the main site’s forums, or network-activate to enable on all sub sites.
If you want both main site and sub site forums displayed on the sub site, set a different root slug and topics slug in the forum settings. After that, both slugs will be known on the sub site and display their respective forums.
BUT! There is a reason why you cannot normally do this and that is because the code base is not really supportive of it. I have monkey patched my way to the goal and I’m doing some evil black magic like hooking ‘query’ and running regular expressions on the query. All of this is only ever done to your forum pages, so be cool, give it a try. If it works for you: great. If it doesn’t: deactivate, relax, look for a different solution (or patch mine).
Profile Note: Profiles are not merged! Favorites, topics started etc show main/sub site only. If you want your users to see all their subscriptions in one place, you’ll have to write some custom code and likely patch my code too.
BuddyPress Note: It does work with network enabled BuddyPress, but they don’t play together nicely because of the BuddyPress/bbPress integration stuff. I have monkey patched this to work. If you do not have BuddyPress installed in /members/ you’ll have to monkey patch my monkey patch, please see the code. And as for the profiles: Same thing as with bbPress profiles.

Multisite bbPress Slave

Make sub sites use the main site's bbPress forums

veppa

bbPress WP Tweaks replaces regular sidebar with forum specific “bbPress sidebar”.
<iframe loading="lazy" title="bbPress WP Tweaks (plugin overview)" width="750" height="422" src="https://www.youtube.com/embed/7LVt41oLscs?feature=oembed" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" referrerpolicy="strict-origin-when-cross-origin" allowfullscreen></iframe>
<a href="https://youtu.be/7LVt41oLscs" rel="nofollow ugc">Watch plugin overview on YouTube</a>
To setup forum specific sidebar you should first select correct forum wrapper file which has sidebar. Then select what action perform with existing sidebar: replace, prepend or append. Then select which sidebar to target with this action. You can also choose “none” option for target sidebar, in that case no bbPress sidebar will not be added to wordpress.
If nothing in forum sidebar then regular sidebar will be shown.
You can choose which forum wrapper template to use in plugin settings page or specify other custom wrapper file that exists in your theme.
Plugin tested with PHP version 5.6, 7.x and up to 8.4.
<h4>Features:</h4>
<ul>
<li>bbPress specific sidebar instead of default blog sidebar.</li>
<li>Change default wrapper for forum pages</li>
<li>bbPress login links widget</li>
<li>bbPress users widget </li>
<li>Login and register links under forum</li>
<li>Show forum description </li>
<li>Forum related columns for users view in admin area</li>
</ul>
bbPress specific sidebar (works only on Classic themes) — you can use different sidebar on forum pages. You can choose which sidebar to replace, append or prepend to. Appending or prepending is useful if you only need to add couple forum related widgets. Custom sidebar will be used in all forum related pages including forum, topic, reply,and user profile pages.
Default wrapper for forum pages (works only on Classic themes) — bbPRess uses wrapper file in your theme in this order: ‘bbpress.php’, ‘forum.php’, ‘page.php’, ‘single.php’, ‘index.php’. First existing template file will be used. You can choose which template file to check first. Most themes don’t have sidebar in ‘page.php’, ‘single.php’ files, if you choose them then no sidebar will be shown in forums. On the other hand most themes ‘index.php’ file always has sidebar. If you cannot see forum sidebar then change this value to index.php in plugin settings (settings -> bbPress WP Tweaks ) page. If your theme has other non standard template files like “page-fullwidth.php” for example then you can use it by selecting “custom” option and writing page-fullwidth.php in text input field. If that file exists then it will be used as forum wrapper.
Font size — Simple switch to fix forum font size. Available sizes: XL, L, M, S, XS.
bbPress login links widget — if you want to display login and register links instead of login form in your sidebar then use this widget. By default bbPress will not show login links to visitors if they want to post in forum. Use this widget instead of login form in your bbPress sidebar.
bbPress users widget — used to display users linking to their profile pages ordered by: 
* most topics with count 
* most replies with count 
* recently active with time 
* online users with time 
* new users with registration time 
* old users with registration time
Login and register links under forum — will add login and register links where “You must be logged in to create new topics.” and “You must be logged in to reply to this topic.” messages shown. By default you will see above mentioned messages with no login or register links. With this options selected you will see under those messages login links and will be redirected back to that topic or forum after logging in using those links. Login and register links can be customized with custom HTML code to match your website design.
Show forum description — By default forum description is shown only on where forums are listed, not in forum page. With this options enabled you will show forum description on Forum page, on sub forums, on topics, on replies. So user will know what this forum about. If it is a product forum then you can add product image, links or buttons to product page, pricing/purchase page etc. This will make navigation between product and forum a lot easier.
Forum related columns for users view in admin area — is must have feature to have for any forum administrator. With this option enabled you will have forum related sortable columns “number of topics”, “number of replies”, “registration date”, “last active date” in “Users” admin page. Topic and reply counts are linked to user profile pages. You can sort by those columns and see latest registered users or users with most topics. Each column seperately can be switched off using “Screen Options” inside users page.
Disable not used features of plugin — you can disable sidebar by selecting “none” as target sidebar. Disable widgets by unchecking relaed checkboxes in plugin options page. All other features also can be individually enabled or disabled in plugin options page.
Attention: Sidebar and Wrapper options do not work in FSE (Full Site Editing) themes.
<h4>Demo</h4>
Check out one of my sites’ <a href="https://veppa.com/forums/" rel="nofollow ugc">bbPress forum page</a>.
<h4>Plugin home page</h4>
<a href="https://veppa.com/bbpress-wp-tweaks/" rel="nofollow ugc">bbPress wp tweaks plugin page & Documentation</a>.
<h4>What is next</h4>
<ul>
<li><a href="https://veppa.com/http-requests-manager/" rel="nofollow ugc">WP_HTTP Requests Manager plugin</a></li>
<li><a href="https://veppa.com/share-button/" rel="nofollow ugc">Native Share Button – without plugin</a></li>
<li><a href="https://veppa.com/improve-pagespeed/" rel="nofollow ugc">Improve PageSpeed</a></li>
</ul>

bbPress WP Tweaks

Thanks a ton!
It solved many problems for us in implementing sidebar to our forum
[ Signature moderated. ]



Very Nice Plugin

Adds bbPress forum specific sidebar, wrapper, widgets, user columns, login links and other tweaks.

Multisite bbPress Slave vs bbPress WP Tweaks