Think of it this way: you are taking your girlfriend to a party with a dress code. It is implicit, and not on the invitations, but, as with any other party, it’s something most guests would like to at least know about in advance. You are not doing her or anyone a service by keeping her in the dark about how the host expects his or her guests to dress.
There are two potential outcomes, both of which I imagine you fear. One is if you explain how tzniut (modesty laws) works, and she still wants to go to the party dressed the way she typically dresses. The other is if she honors the dress code, but feels uncomfortable the whole night because she has muted some part of herself. Presumably, the reason you want to bring her to this party in the first place is because you are trying to integrate her into your community. Either of those responses makes this harder, but there are ways to work through it.
Whatever she decides to do, you have to support her without reservation. Ultimately, bringing her to his party is about making her feel welcome among the people you grew up with so it’s in both of your best interests if you set her up for success. Be great about making introductions and talking her up. Be effusive. Make sure she knows how glad you are that she is coming, and that your friends know how excited you are to introduce them to this special person in your life.
If she chooses to dress how she usually does for the occasion, some people may be unpleasant about it. But those people were going to be unpleasant about the fact that you are dating a non-Jew, anyway. They would just be using different ammunition. People who care about you will be glad you have found someone who makes you happy, even if they express cautious concern about other things.
Ultimately, tzniut are about humility. If you can approach this situation with the humility to let go of your own fears and support your loved ones (both your girlfriend and the friends you are introducing her to), I have no doubt the evening will be a success.
Benjamin Kamine is a New York-based stage director, primarily focused on new works. His recent credits include productions at The Goodman Theatre in Chicago, The Flea Theater in New York, and development work with companies all over the country. He was a 2014-15 Fellow at LABA: A Laboratory for Jewish Culture, and is currently working to expand his knowledge of Jewish texts.