You can either create an account on the website, or register after receiving an invitation by mail from an organisation.
On the main page you can join as a user and follow the instructions:
Then complete the registration form with your information.
After entering your details and password you will receive an email from email@example.com, follow the link to validate your account and log in.
You should have been invited to join an organisation by receiving an email from firstname.lastname@example.org.
Just follow the link, enter your details, and validate your email. You will be logged in your organisation secured portal.