The Signed-up segment works hand in hand with our FS.identify call.
FullStory considers "anonymous" users to be those that don't have a unique user ID assigned via FS.identify, and "signed-up" users to be those who do have a UID.
Note: It is possible to pass user information to FullStory, such as displayName or email address, and still have the user be anonymous. They will not be considered "signed-up" until the user ID has been assigned. This unique User ID is what we use to coalesce sessions for a single user across multiple browsers and devices.