Because of the legal implications. RealLegal takes reasonable measures to ensure that password information is protected. If you could enroll someone, you would also know their password, and that would compromise the security of the system.
A person must enroll themselves by completing and signing the RealLegal Electronic Signature Service Reporter/Participant Enrollment Form, having their signature notarized, and then scanning the form and returning it to: rl-esignatures@thomsonreuters.com . A representative from RealLegal will contact the person to verify activation of the Signature service.
In summary, for anyone to sign transcripts, they must first enroll with E-Transcript Signature service and before you can add another person to your billing account, they must enroll in the E-Transcript Signature service.