Possible Clues:

• Acts as a sub (for)
• Observes, with "on"
• Takes part in demonstration but doesn't stand out
• Audits, with 'on'
• Attends a meeting, with 'on'
• Attends as a visitor (with ''on'')
• Audits, perhaps
• Takes part in demonstration - it's included in offence
• Takes part in demonstration, but doesn't stand out
• Takes part as a visitor