An organisation that uses TrustLoop will supply you with a link to their registration form. (Alternatively, you may receive an email asking you to take ownership of a member’s profile that they have created for you.)
By following this link, you will be guided through a process of logging in to your TrustLoop account (or creating one if you don’t have one already) and either accepting ownership of the pre-created profile or completing a registration form to register a person with the organisation.
Here is a tutorial through this process.