They are completely different things.
A registered guest is for anyone who does not have a spouse or domestic partner to pick 1 person to be able to travel on their pass for the year at no charge. The value of their travel will be included on your W2 as earned income and you will be taxed on 10% of the Y fare point to point for each of their flights as the value of their travel, but there will be no out of pocket expense.
On the other hand, companion pass, buddy pass, etc are not added to your income, but they have to pay up front. It is currently (on the East side) 10% of the full Y fare plus taxes and you get 8 a year. In the near future these will be a fixed amount based on the mileage (plus intl taxes if any.).
West side still has flexis floating around for a while so the value is paid for by the person using the ticket and it depends on whether they are doing guest or companion travel.
You can still have a registered guest for the 1 person free travel for the year as well, but I believe the registration period for this year was up back on the 31st.
Anyone with a better explanation is free to chime in. 😉