To add collaborators to your space, there are a few key steps:

In the “Linked Organisations” tab on the right, you will see all the spaces you’re a part of, and your role (admin/ editor). If you aren’t an admin, find out who within your organisation is for the space so they can add collaborators, or make you an admin.
Under the organisation name at the top, you will then see “Organisation settings”. Click here.
Click “add team members”, enter the new collaborator's email address, and then “send invites”.
They should now receive an email with a link to join the space as a collaborator.