To send texts from Clearstream, you first need to add subscribers (contacts) into your account. There's no limit to how many subscribers you can have, and they can be added to your account a few different ways.
CSV Import: Import a spreadsheet of phone numbers from Excel, Google Sheets, etc. and optionally add their name and email address.
Keywords: Let people text a keyword to opt themselves into your account.
Signup Forms: People can fill out a form on your website and select which lists they'd like to join in your account.
Note: You must have verbal or written permission to text each person before adding them as subscribers in your account.