Group forums are separated by groups. Group forums require users to be part of the group in order to use the forum. This method can be good if you want to restrict usage of specific forums to specific groups of people.
Site wide forums is one centralised forum where users do not have to be part of a group in order to use it. This method is more like a regular forum.
When installing buddypress you can enable the forum component (doing so will give you the built in function for installing bbpress in either form), and later remove it in order to use site wide forums with the /forums slug.
Once its installed, delete the page “forums”, ensure its removed from the page trash.
Go to Buddypress > Components, and uncheck “Discussion Forums” and click save in order to disable to buddypress (group) forum component.
Double check that bbpress is using “forums” as the Forums base.
Go to permalinks and save to update.
Navigate to /forums on your site and you should be able to see the site wide forums. You will have to manually add forums to the main menu.
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Its unfortunate that BuddyPress was set up this way, I can see users wanting to have site wide forums more so than group forums, and will want them to be on the slug /forums. A better way would have been for them to use /group-forums or /user-forums for group forums, and /forums for site wide forums. Lets hope an update will rectify this.