If your account owner has invited you to join their Textline team as an Agent, you will receive an email or SMS notification that will walk you through the remainder of your account creation. Your email address will be your login.
Another way to join an existing account is to sign up with a claimed email address domain that has been claimed by an Admin of the existing account. See how to do this by clicking here.
Please note that if your email address already exists in the Textline system - whether you've created an entire account, or been invited to an existing account - you will not be able to use it to join another account. You will need to either change the email address in the existing account or have an Admin in that account delete your login in order for your Admin to invite the correct email address to the new account.