Party Crasher in 007 First Light is the Ocean View Bar entry challenge in Ellis White’s mission route: get the flyer from the two women outside the bar, then show it to the guards to walk in.
Party Crasher Challenge details

How to complete Party Crasher
Step 1: Head to the Ocean View Bar
From the resort, go down the stairs and keep moving southeast until you reach the Ocean View Bar entrance.

The route is along the rock steps and down toward the blocked-off bar area, where two guards are posted.
Step 2: Trigger the flyer opportunity
Speak to the guards first. After that, two women will approach and sit behind you, and they are the ones carrying the invitation flyer you need.

Step 3: Stun and pickpocket
Use either the Phone Dart or Laser Strap to stun the women.

Then pickpocket the flyer while they are disabled.

This is the cleanest way to avoid a messy alert state.
Step 4: Show the flyer
Return to the guards and present the flyer to gain entry.

That action completes the Party Crasher challenge and moves the mission forward.

Mistakes to avoid
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Don’t rush to the guards before the women appear behind you; the flyer only becomes available after that setup.
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Don’t rely on brute-force movement here if you want the challenge cleanly; the guide path is built around stealthy interaction, not combat.
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If the women are not stunned, the pickpocket window is harder to use safely, so use the gadget first.
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If you’ve already entered the wrong area of the resort, backtrack to the Ocean View Bar approach instead of searching randomly.
Completing Party Crasher is the fastest way to get into Ellis White’s party area and keep the mission chain moving without alarms, which also preserves the clean setup for the next objectives in the same run.