The scale determines who you are based on the weight reading. If your weight varies significantly compared to your last reading (something like +/- 10 pounds), it won't know you are so it will assign your reading to Guest.
You can fix this by manually assigning that weight reading to yourself. Once you've done that, it will learn what your new weight is and any new readings that are within the buffer range will be assigned to you.
To reassign the weight, log into your fitbit account in your browser. From the dashboard, select the settings menu in the top bar (the gear icon on the right) and select your scale. From here, you will see all of the readings from your scale and who they are assigned to. Some of them will be set to Guest. Just select your initials from the list on the recent reading to reassign it to yourself.
