Hi @Oliver,
Thank you for sharing this question with the Klaviyo community.
Klaviyo does not currently receive any information from WooCommerce to distinguish between guests and registered users. One strategy to distinguish between these groups is to add them to discrete Klaviyo lists from WooCommerce, this can be done through a list upload or using our V2 Lists API (specifically the subscribe endpoint: https://developers.klaviyo.com/en/reference/subscribe).
I also recommend reaching out to WooCommerce to see if you’re able to “bake in” an additional radio button at checkout to gather/tag whether a user is a guest or registered. Currently, we use a checkbox in the plug-in to determine if a user accepts marketing, but there is nothing by default that captures their registration status. If we can gather that information, then we can segment users (based on properties about someone > registration status = X) and/or set up flow filters to exclude or include users based on their status as a guest or registered user to achieve your goal.
I hope that is helpful!