I don't think you can accomplish what you want with Meraki Authentication. One approach would be to use RADIUS authentication for JumpCloud. You would need to create a RADIUS configuration for each site, assign your guest users to a group and assign the group to the RADIUS sites.
JumpCloud can import users from Google Workspace, Microsoft 365 and Active Directory.
If you are only using JumpCloud for RADIUS authentication, it can quite inexpensive for the value.
PM me if you want to know more about JumpCloud or would like pricing.
Dave Anderson