After you've selected a data set, you'll want to choose which records should be emailed. If you want to send to every record in your data set, simply click the Invite All Records button at the bottom.
You can, however, select only some of the records by using the checkboxes on the left. Also, don't forget to use the search box near the top to narrow the list of contacts. If you used tags or other information to divide up your records list, you'll be able to find only certain contacts by using the search box.