In order to import subscribers, go to the Messaging & Workflows Tab -> Click on Lists & Subscribers -> then click the More Button -> click Import
This will allow you to import users via a .CSV file format and map the fields to the properties within the CRM
You can also subscribe users in bulk from your current Contacts tab by filtering/selecting them and clicking on More. Then select Add Contacts To List and select which list you would like to add them to. This will subscribe and add the contacts to the selected list.
