Using the user workflow settings it is possible to have new user registrations subject to approval by an administrator. That way you could open registration to a Jiglu site but only allow those users with the right credentials to have access. However, it's possible that for users with certain email domains you're happy for anyone to register and would prefer not to have the extra work in approving them. To allow this you can configure pre-approved domains in the workflow.
Go to System settings and choose the User workflow option. In the Pre-approved domains setting you can list all the domains for which users will automatically be approved. For example, to allow someone with (Address removed), (Address removed) or (Address removed) to be pre-approved you would use 'example.com,example.net' for the setting.