How do the pacing buses work at big races like the Comrades Marathon

Today’s question comes from Siobhan Macleod.My hubby and I are both new to the Comrades Marathon and will be running our first Comrades this year – how do the timed ‘buses’ work, i.e. I have read that there is an 11 hour bus (I assume it’s a trained runner leading a group) and 12 hour bus etc where runners can stick with a guy/group to try and complete Comrades within that time frame. Is this true and how does one join or run with a bus?Thanks, Siobhan