There are two ways to subscribe people to your newsletter. You can either subscribe subscribers individually or subscribe in bulk.
In order to subscribe an individual, you can go to Subscribers and then click 'add' to enter their information. In order to subscribe in bulk, click 'export' to download .CSV file. The file should download with seven columns: first name, last name, email, active, subscribed, created and updated. Fill in the columns with each subscribers information and make sure there is a 1 in the active column. Save the .CSV file and then re-import into the platform.
This video will walk you through both ways of subscribing.