If you go into your own account settings and add yourself to the Beta Tester group, the forum will hide all boards from you, even if you still have the Registered User group as secondary. This is a SMF permissions bug that needs to be resolved in order for Beta Testers to continue using the forum normally like regular users do, in addition to any extra permissions and/or boards they may have been granted along with the group.
You can also just simply do beta testing with out signing up as a beta tester.
I’ve done that for the last 8 years. 
I’ve done it before too, and figured that was the case, however the forum group being available while being bugged is quite an issue in itself that should still be resolved.
After joining the Beta Tester group, the Registered User group must be set to Primary group.
That shouldn’t be the case in a SMF 2 forum, though. The same permissions should still apply regardless, which is why it doesn’t make sense to me. That also seems like an odd issue to allow anyone to have access to without any detailed explanation to go with it. 
Sorry to bump this super-old topic, but this bug still exists, (although it isn’t nearly as bad as it was before). When the primary group is set to Beta Tester, several forum sections completely disappear from view. If the permissions for the forum are set up correctly, this should not be happening. SMF is designed, by default, to only grant access, not remove it. To remove access means either a modification, or a custom setting, is being used which explicitly removes board visibility for that membergroup.
Can someone please fix this? It is extremely annoying having to constantly switch between membergroups to access the appropriate boards. This also affects email notifications for topic replies, if subscribed, depending on which membergroup is set as primary at the time.