To activate your BriefBuilder account, you must have received an invitation mail in your mailbox.
In the invitation mail, you will find your (automatically generated) username and an activation link that you need to use to set up a password.
If this somehow does not work, the following may have happened:
Activation link has expired
The activation link remains valid for 14 days after you have received your invitation mail. So, if you try to activate your account 14 days after you have received the invitation mail, it is not going to work.
Solution: ask your administrator to send you a new invite.
Wrong auto-fill in browser
Another issue can be that you already have a BriefBuilder account for another BriefBuilder environment. In that case, it can be that your browser ‘auto-fills’ the username field with the wrong username.
Solution: clear the username field and copy-paste the right user name into it: the one that has been sent to you in the invitation mail.