I agree with Rob that ClearPass is the best solution for managing guests across multiple Instant clusters.
But if you really want to use AirWave push local guest user accounts across those clusters, there is a way to do it. The workflow would be roughly:
1. On one Virtual Controller, add the guest users.
2. In AirWave, find the group that the VC is in and go to the Templates page.
3. Fetch a new template from the VC you updated. Since a VC's internal guest users are in its running config, the users you added in step 1 will be in your template.
4. Assuming your other VCs are in the same AMP Group, put them in Manage mode and have AirWave update their configs.
An important thing to remember here is that when AirWave manages a VC's config it manages the whole thing, so this is only practical if you have a standard config across the VCs.