Unfortunately it is not possible to create this type of alert in Change Auditor. CA cannot enumerate the group memberships (to determine who is a member of group A) nor can it correlate the 2 events together (User from group A memberOf attribute changed and group B member attribute changed).
You can create an alert for the list of users in group A so that when any of the users is added to group B you will be alerted. The search used would need to be maintained as the list of users would be static:
- Create a new Search
- For the What tab add Group A and select "Members of this group" for the scope, Or, add all of the user objects from the group, selecting just Object for the scope
- On the What tab also add the User member-of added Event Class. Under Restriction select 'Where the new value contains the following text:' and enter the name of group B