Possible Clues:

• "Flash" gatherings
• Surrounds in a rush
• Hoodlum observed taking part in riotous assemblies
• Flash ___ (impromptu gatherings)
• Crowds of thousands round cleaned-out banks
• Greets with too much enthusiasm
• After-Christmas sale shoppers, e.g.
• Overwhelms
• Swarms around
• Swarms on