In this dialogue, two female agents (two of the most conscientious officers of the team) are having a phone conversation from two different countries. The one who is baking the cake is inviting the other to come along for a special occasion in a forward operating base in Afghanistan where a suspicious mole is supposed to arrive and provide important information to the investigation.
– Don't be so literal, everyone likes cake. It's not too late for you to come, you know, it'll be fun.
– No, I don't want to be a straphanger. It's your show. You were the first to see the potential in this.
I guess 'straphanger' seems to be used in a metaphorical way here, but I didn't understand it in a clarified way. I'd like to get further clarification because it appears to be used in an interesting way.
(Film: Zero Dark Thirty - United States, 2012)