Hi Ali,
The only way to report on contact groups from an event would be to go to Promotion & Communication > Email > Invitation Lists > Click Advanced Search > Edit the Contact Group dropdown. If you only want to pull Accepted registrants, change the Invitee Status dropdown to Accepted. If you have multiple Invitation Lists added in your event, repeat this for each Invitation List.
I've submitted your feedback to our Product Management team to consider implementing in a future release of our product.
Best,
Julia