wpForo allows you to manage accesses to forums and topics. But the whole forum page is just a regular WordPress page and you can use any WordPress plugin to restrict access to certain page: https://wordpress.org/plugins/search/restrict+access+by+user+role/

Once the page access is restricted using some plugin you should also care about direct accesses to forums and topics. So you should set "No Access" to all usergroups you don't want to see them. You should edit all forums one by one and set proper accesses to usergroups.

Here is the instruction: https://wpforo.com/community/faq/wordpress-user-roles-wpforo-usergroups-and-forum-accesses/#post-39664

Ideally, if someone who wasn't logged in (IE: a public user) went to the forums permalink (which I think by default is http://mysite.com/community/), it should re-direct them to the Wordpress login screen asking them to login.

Thanks.