Hi,
Event field mappings manipulate data on the event record, not the alert record. You could update the additional_info to contain the assignment group sys_id, and then map it as part of the event rule. But you would need to do the mapping per-rule.
Another solution, assuming you're binding the event to the cmdb_ci_batch_job CI, you can use assignment rules Introducing Alert Assignment Groups in the Washing... - ServiceNow Community
