Is there any way of just allowing users from existing organizations to create accounts in Guide?
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You can disable 'Anyone can submit tickets' option in the Admin Center. By doing this, only added users in your instance will be able to sign up for an account in Guide. You need to manually import users via bulk import or API and I recommend enabling Also send a welcome email when a new user is created by an agent or administrator option to send a welcome message.
When you receive a support request from an anonymous user, the request is added to the Suspended Tickets view. From there, you can review the ticket and either delete the request or recover it.