This is why I suggested that you could do away with the guest data type altogether and put all the fields in the User data type. Only users with a role of Guest would need to use these fields. Without knowing the details of your application, I would say this is likely what I would do.
Still, even with your current approach, you can add a user field to your guest table and when you create the user, you add that user to the guest record. That way your repeating group can be of guests where user = Current user.
In general, I would suggest that you take a pause and take one of the several courses available, if you haven’t done so already, just so that you don’t go down a route that will hurt your application down the line. It’s easy to make changes in Bubble, so maybe it isn’t such a big deal but better to get your data structure right from the start if you can.