The most common way of getting started using Bimsync is by invitation to a project. You will receive an email with the subject "(some person) has invited you to (some project). The name of the person and the persons email will be visible in the invite:
To accept the invitation you click on the "Click here to accept invitation" link.
When you click on the invite you are taken to Bimsync where you are asked to sign in and accept the invitation.
If you don't already have an account you will be asked to sign up for an account at Bimsync. Follow the signup procedure. Next click "accept invitation"