Go into your Lists & Subscribers section in the Emails & Automations tab and click on More -> Import. This allows you to import users via .CSV file format and map the fields to the properties in the CRM.
You can also subscribe users in bulk from your current Contacts by selecting them and clicking on More. Then, click Add Contacts To List and select the list you want to add them to. This will subscribe and add the contacts to the selected list.
