Caucus confusion

On Left Center Left, Chris Cagle reports on the mystery that was the Ward 10 Democratic caucus for Mission Hill:

I felt like a French peasant in the provinces while revolution waged in Paris - vaguely aware that something was happening elsewhere in the state, but unsure exactly what it could possibly be. ... I'm mystified by these tallies that people are giving for Patrick vs. Reilly's delegates, or news that Reilly led in Boston. How do they know? Is it something that the campaigns themselves tally? Because there was barely a word at my caucus of how anyone would vote at the nominating convention.