As a new user being added to an event, you will receive an email from an Administrator inviting you to activate your account. Within this email is a link that reads Get Started - follow this link and you will be taken to the activation page, where you will be prompted to choose your own password. When you are happy to proceed, simply click on the Activate Your Account button.
Alternatively, if you are invited by an Administrator while they are creating a conversation or comment, you will receive an email notification saying '**USERNAME** has left a comment for you' or '...added a file for you'. When you choose to view the activity by selecting the link in the email, you will be taken to the activation page before you can access the application. Simply choose yourself a password and click on the Activate Your Account button and you will be taken directly to the activity in question.