We all have individual Warhorn accounts for table sign-up. People tend to make their own events; among other things, it helps GMs regulate table size and screen their players.
IronHelixx and Roll20 PFS have some "communal" Warhorns (based on what I've observed with the multiple GMs). You'd have to speak with the event organizer in order to add sessions.
You can also team up with another person who runs online and use the same Warhorn Account. The creator would have to give you admin privileges in order for you to update sessions.