@Elmira P - Anonymous guests would be considered as “not signed in to a Miro account”. If you need them to be anonymous, you would use Visitors (also known as Public) access: https://help.miro.com/hc/en-us/articles/360012524559-Collaboration-with-visitors (Note: Signed in users can also access boards using this method.)

If you have a Consultant, Business, or Enterprise Plan subscription, you would also have access to the Guests feature, which allows you to share a board with specific Miro account holders without enabling Visitors/public access to the boards - more on Guests access here:

https://help.miro.com/hc/en-us/articles/360021415119-Collaboration-with-guests