In the APgroup settings, you should see the below configuration in the radio options. The UI might look a little different in some respects...the screenshot below is from a 5.0 beta version. Look for the WLAN Group selector. This is where you select the WLANs that will be advertised on the radios of that APgroup. If you don't see the WLAN group setting, double check that you are editing the APgroup instead of the Zone. And if you still don't see it, let me know what build you're running.