Joel is enamored with Rosh, as Daniel was at the beginning, because he sees the hillside bandit as representation of rebellion against the Romans. He believes that Rosh is fighting for the cause of Israeli freedom, and so is willing to spy for Rosh if needed. In Chapter 16, Daniel conveys a mission from Rosh for Joel to gather the names of rich villagers who are going to attend a banquet with the half-Jew ruler, Herod.
"What he wants to know is what others will be there. The names of all of them, and the day and the time when the banquet will take place."
Joel nodded. "I can see why. It would give us a good idea which men would be against us. Any Jew who would eat at the tetrarch's table--"
"That's it. We need to know our enemies as well as our friends."
(Speare, The Bronze Bow, Google Books)
In Joel's mind, they are gathering information so that when they rise up against the Romans, they know who they can trust among the wealthier and more influential villagers. This would allow them to quell internal dissent and present a strong face. However, Daniel discovers through gossip that Rosh's real plan had nothing to do with the cause; Rosh instead visited the empty houses of those villagers and stole their wealth. Rosh cared nothing for gathering intelligence; he only wanted to find out which houses would be unguarded so as to line his own pockets.