QUEST: Recruit the Adventurers at Red Flagon!
If the adventurers go to the Red Flagon Inn, they see that it is simply bustling. If they can act out an epic adventure they have had so far, with five DC 10 Performance checks, they can try to spar with a few drunk adventurers, numbering ten in all.
If they win, they can use a DC 10 Charisma (Persuasion) check to recruit the adventurers. If the adventurers roleplay this encounter, the get an advantage on the rolls,making them a DC 5.
This way, they can gain 50 adventurers for the siege. Otherwise, the adventurers are unimpressed and will not believe them, and will leave.