In Crispin: The Cross of Lead, who is at the White Stag tavern?

Through bribery, Crispin convinces the guide, whom the widow arranged to take him to the town walls and safety, to lead him instead to the White Stag where the conspirators are meeting. So, the conspirators are at the White Stag. Crispin approaches them for help in arranging an escape for Bear, who is being held elsewhere. The conspirators decline because the town is heavily guarded but offer to escort Crispin to the town square.

Once there, Crispin discovers John Aycliffe and convinces him at knife point to release Bear. Had Crispin not gotten to the conspirators at the White Stag, he never would have encountered Aycliffe and Bear would never have escaped. Of course, the escape leads them right into another ambush or two, but that's for another question.