Other than painfully moderating signups, is there another way to secure a help centre? say by domain?

Can domain access be added to the permissions page?

My use case is for an internal helpdesk. Another use case is rather than help center's by brand one could setup help centers for important customers.

What I've done so far is turn on multi-branding and enable unique users. Now what I want is to restrict this particular help center to employees only. I've already disabled sign up and enable register to access and register to submit tickets. I've left on auto-invite since I used an email group and restricted group email to organization only and to bounce other emails. What else can I do? 

I thought maybe using Zoho federated login, but again, how do I restrict this to a specific domain? What if other Zoho users stumble upon this help center? It doesn't take much to figure out it's hosted by zoho and any one can go create an account and gain access.

Can this be achieved with the way things currently are?