Some pics from ZD might help (or not).
[With only a few devices it seems madness but when you have dozens or hundreds breaking APs and WLANs into groups gives you much greater control]
ZD gui, Configure, Access Points:
Create AP (antenna) groups

then add APs to that new group

Create WLANs

and add them to WLAN groups

So now you can go back to the AP group and select which WLAN group it will use - greyed out box at bottom.
AP groups contain APs and use WLAN groups to know what WLANs they should broadcast.
WLAN groups contain the individual WLANS and they use ACLs to allow clients by MAC. Simple, tee, hee. I need to rest.
Hard to get your head around but it all makes sense...eventually.
Max