I (19M) asked my girlfriend (19F) to not do an activity with people from work and she seems upset. Should I stick to what I’m saying or back off?

My girlfriend and I are in a long distance relationship currently. She mentioned that she wanted to go do an activity for 6 people (it requires 6 no more, no less). I asked who with and she said a man she works with and a girl she doesn’t work with are going on a date, so her, her female coworker, and two male coworkers were going to “crash” their date and do this activity with them. I have never met these coworkers and expressed that I am uncomfortable with her going on what in my mind is a triple date. I’m asking for advice on whether I’m overthinking it or if this situation really is something that is valid to not want her to do. I am by no means controlling and she can go if she’d like to, but I did overtly express that I believe it is a triple date. Should I stick to what I have been saying or just back down and tell her I’m okay with her going?