Rob, there are no default settings to use forum within Groups only.
We can suggest you to move Forum to hidden pages in Admin Area>Pages&Menus>Pages&Menus section. You can find more information how to manage menu items and pages visibility here:
https://docs.oxwall.org/user-manual:pages-and-menus
But if you want totally hide all forum links and leave "general chat" for groups only, this is considered as a custom code modification.
If you are not familiar with the plugin development or code modification, it would be better for you to find a developer who can help you. You can do it here:
http://www.oxwall.org/market/specialists