The SendGrid username is a credential that you use to login. Your signup page requires username and password to login.
You can find your account Username in the Account Details (Settings > Account details > Account tab) section of your UI.
Please see the screenshot below:
Only an Admin or a Teammate (provided they have enough access or permissions to view the account details page) can login to the respective SendGrid account.
Here's more information on SendGrid teammates permissions - https://docs.sendgrid.com/ui/account-and-settings/teammates
If you are signed in and would like to find the username for another teammate in your account, you can find teammates, and their respective usernames, listed in the Teammates page of the account (Settings > Teammates). In order to view these you would need to be signed in as an admin or a teammate with the proper permissions to view this page.
If you are not an admin or teammate, you can also find your Twilio SendGrid account user ID on your invoice in the Invoice Summary section.
