In order to use the Import Contacts function you will need an active Integration channel other than Web.
(You cannot send outgoing messages to Web contacts unless they visited your website first - in which case, these contacts would be available. /Promptchat would have placed a cookie in the users' browser in order to allow sending follow up messages to this browser upon a follow up visit/.
Once you have an active Integration (SMS, WhatsApp, Viber, Telegram, Instagram, FB Messenger) You can pick the relevant Integration Channel and import Contacts from a .csv file.
Best practice is to add Tags during the import and assign a department too. The date and time of the import is automatically added during the import.
Simplest way to set up an Integration is by the SMSchat SMS gateway - You will need an Android phone. Visit: https://smschat.org -> https://forum.promptchat.com/d/22-sms-integration