If you are looking for Assignment Groups Incident records has been through I would suggest you to report on Table Incident Metric & add filer as Definition is Assignment Group. This would help what you are looking to achieve.
If you are looking for Assignment Group for records other than incidents then you need to create Metric & then get it used for database creation & then finally report on the same. Links below should help.
Metric creation
Database for metric
Thanks,
Jaspal Singh
Hit Helpful or Correct on the impact of response.
