This is rather a very specific extension with predefined features, but I will publish this for free so if you see a potential in it, you are free to use it.
Allows admin to instantly create a group, forum with the same name and only users within that group can see the forum. It also adds a special page where users can subscribe or unsubscribe from those groups and a special page where users can see a list of topics from groups they are member of.
Extension repo: https://github.com/senky/phpbb-ext-groupspp
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