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.

Why do Joel and Daniel think Rosh wants the banquet guest names in The Bronze Bow?

The answer to this question can be found in Chapter 16, in which the latest orders of Rosh are given to Daniel. In order to carry these orders out, he needs Joel's help, and so he goes to the city to speak to his friend. Rosh has told him that King Herod is hosting a special banquet to welcome a special legation from Rome, and all of the rich and powerful in Capernaum will be present at this banquet. Rosh wants to know the names of those who will attend this banquet. Note what Joel says in response to this request:

I can see why. It would give us a good idea which men would be against us. Any Jew who would eat at the tetrach's table--

Joel therefore assumes that Rosh wants to know who will attend the banquet in order to discover which members of Capernaum society can be depended upon to support the resistance and which are in league with Rome. Of course, as Joel and Daniel discover later on in the story, the real reason is that Rosh plans to use the banquet as an opportunity for robbery, as he will know which rich residents will be away from their homes. This is actually a significant event in the novel in terms of charting Daniel's growing disillusionment with Rosh and his realisation that Rosh is not the resistance hero he thinks he is, but rather nothing more than an opportunistic individual who seeks to personally profit himself alone and is not interested in the liberation of the Jews.