She is bugged in the registry (It has her still in the cell). To fix exit your game and go into the registry, (right click windows start button, run, type "regedit" hit enter, go into folder "Someguy" then "Something Unlimited")
Find the slot with your save game, i.e. suSlot01, suSlot02 or suSlot03, and look for the i.e. suSlot02_zatannaP-h as shown below
View attachment 2239571
to fix it, right click it and choose "Modify Binary Data"
To have Zatanna not greyed out, delete the numbers 63,65,6C and 6C..leaaving just 00, hit the OK button. My Save game for Slot01 had Zatanna ready to be put into either GS or MB as the _zatannaP_h was already set to 00
View attachment 2239620
Go back into your save game and she will be ready to be assigned