Usually you have to do this at the begining. Subscribers can be imported via copy/paste or via file upload.
Go to: Subscribers » Import subscribers
If you have less than 1000 subscribers you can copy paste directly from Excel or CSV.
If there are more subscribers, it’s recommended to upload directly from the file. File must be Excel file (XLS / XLSX) or CSV.
We recommend you format your file before uploading. Please download this file with example data: SubscribersImportExampleFiles-Newsman.zip.