I set up a new user against my Azure account, under the Active Directory area of the management portal. This user has been set up as a Global Admin and is someone with an existing Microsoft account, hence that was how I set them up.
They are, however, unable to log in either to the management portal (error: no Azure subscription found) or Cloud Explorer (same error message).
We wish to use our Storage Account for development projects, hence the requirement for more than one distinct user to access. Have I missed something?