I've spent some additional time working on this today and found some helpful info:
When you are getting registrant info from the Event Registrants Collection, the guest information is not included in the API response. This is primarily because the content of the response could be exceptionally long, as each registrant is capable of having tens of guests if an event is configured to allow that.
If you request the details for an individual registrant by ID, then the guest information will be returned, and will also be serialized in the .NET SDK. I have thus far only tested this on the current version of the SDK, but I am also happy to help look into any issues that you may be having with an older version.
Lastly, I discovered an issue in the SDK where having a large number of registrants results in the API paginating the registrants collection and the SDK is not properly configured to handle this pagination. I will be working with the team here to get this feature added in to the SDK in the very near future.
If you have any questions or if your experience is different from what I've described, please let me know!
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