You can use the Message Open Report to identify constituents that have opened an email message and then build a group from those report results. Follow the steps below to do this.
  1. Select Email > Email Campaigns
  2. Select the Campaigns tab
  3. Click Manage under the Actions column for your campaign
  4. Select the Reports tab
  5. Select the Message Open Report from the dropdown menu
  6. Click Go 
  7. Add the message to the Your Selections on the right
  8. Click Next
  9. Click Submit Report 
  10. Once you run the report it will be in the results area
  11. Click View
  12. Click Add results to group
  13. You can create a new group or add the results to an existing group
You now have a group of constituents that opened a previous message which you can use as a target group within your next email.