She is certainly at the party according to Shakespeare. Having no direct contradiction in Luhrmann's version, she is almost certainly at the party.
Romeo does not meet Rosaline at the party, again, as far as Shakespeare is concerned. Romeo sees Juliet scant moments after arriving at the party and is so bewitched by her that he's totally over Rosaline in a heart-stopping heartbeat. Romeo probably wouldn't give Rosaline a second look after he sees Juliet.
If we're assuming Rosaline is at the party, I suppose it's possible she's in a crowd shot. But this one there's no source material or "word of God", so it's impossible to answer.

I wish I could ask Baz Lurman, because then we could see who she was talking to

Presumably
Don't Know
Presumably