To elaborate in case you are unaware since users can belong to multiple groups the permissions in phpBB are stacked. A users final permissions are the aggregate of yes permissions minus never for every group they belong to.
No- better defined as not set here.
Yes - permission is granted and overrides any no permission in other groups.
Never - Never is always never and overrides yes.
Use the registered users group as your base permissions you want everyone to have using yes and no. The only time you would use never permissions here is if you want to to remove them from everyone.
Assign yes permissions in other groups you want to have more features to override no set for registered users group.
Assign never permissions to take away permissions set to yes in registered users group, e.g. NRU group.
Clear as mud, right?