The Group tab in the Delivery section of reporting can be a great tool if you need to send a follow up campaign or simply forgot which group you sent to.
To do this go to Reports and find the campaign in question. Click into the campaign then click the Delivery tab.
Once in that tab, go to Groups
You will then see the group(s) that your campaign was sent to along with click rates, recipients in the group, contacts, bounces etc.
DISCLAIMER: Please note, when sending a campaign, remember to click the tick box beside each group on the main Group tab as shown.
If you click into the group and tick the top box, as shown below, the group will not show when you follow the steps above as it will count it as manually entered contacts.