I created an account and have been given access to the organisation but do not seem to have access to the other organisations. Why is that?

With LimaCharlie's granular role-based access control you can be granted access in one of two ways:

You'll want to ask the person who granted access if they added you to the individual organizations, or if they'd set up an organization group. Either method works, but they'll have to ensure that either you're added to each organization individually, or that they set up a group.