1.
Guest can not access the forum or view posts in forum

Admin's CP -- > top menu --> Users --> User Group --> System Group - Guest --> [Detail] -->

Access Level: 0

And ,if you set the guest -->View Forum Statistics: no ,then the guest cant see anything about your forum until they register.


2.
And if you want one or some groups can access a forum ,eg,you want only moderator and Adminstrator can access this forum:

Admin CP. --> Forums --> edit forum --> choose the forum which you want only moderator and Adminstrator can access,

--> [edit] --> Perimission (No Select to default) :    choose the moderator and Adminstrator all below option

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