In the reporting section, you will be able to see who has opened your email, you may then wish to add these into a new group to send further communications to, luckily you can!

To do this you need to go to the 'activity' area within the email report and then filter the open box to 'greater than or equal to', and then type 1 into the text box, the report will then update to include everyone whose opened the email. Now go to the bottom of the page and you should see an option to 'add to a group'