Go into your Lists & Subscribers section in the Email Marketing and click on More -> Import. That allows you to import users via .CSV file format and map the fields to the properties in the CRM.
Choose Import, and choose a list:
Go through the import process to upload a CSV file and match the columns to the CRM properties before finally importing.
You can also import people from the Contacts tab, and then highlight them all and add contacts to as list:
You can also subscribe users in bulk from your current Contacts tab by marking them and clicking on More. Then select Add Contacts To List and select to which list you want to add them. This will subscribe and add the contacts to the selected list.