But this isn't the 'party' that they refer to at the start, as that is the party at the apartment that you can see them preparing for.

That doesn't mean it makes sense. The way Nick says that line - clearly off-camera, makes it looks like ADR they added in after because they realised the plot made so little sense and they were trying to backtrack. How could he know what screening she would go to? And the main point is that, why did all the guys act…

Is anyone going to mention that the entire plot made no sense whatsoever? Nick had no idea what to do for Jess' birthday, and the whole day was running around and phoning the other guys, clearly showing that they were NOT in on an elaborate plan to hijack her cinema screening. Then just as everything has gone…