BuddyPress.org

just look at the core files bp-groups.php and the function groups_setup_nav()

from there you’ll see how bp sets the current access for most subnav items
look for this line -> // If this is a private or hidden group, does the user have access?

which is referenced on line 206/7 for the Forums subnav. What you can do is remove the subnav ( bp_core_remove_subnav_item ) then re-add with the param user_has_access calling some function to determine the access level for your given requirements. (hook your remove/re-add on the action groups_setup_nav )