Your Sendgrid username can be any string of characters that works for you! It can even be in email format if you’re comfortable with that.
If you are unsure what your username is, consider checking your existing code or SMTP integration, as your username will probably be in there plain as day.
If you still can’t find your username, get in touch with support, and we can help you track it down!
SendGrid also allows you to create multiple sets of credentials for your account. You can create a set of credentials for sending mail, website access, API access, or any combination of the three. You can setup and manage credentials here.