I don't believe you've done anything "wrong", especially because, as you said, you did not leave on a bad note.
You shouldn't feel as though you are obligated to become best friends with her best friends. In fact, its almost better that you can maintain a relationship while still being able to have seperate groups of friends.
From the sound of things, this isn't bothering her so much as it is you. Since you say you are not jealous of her spending time with her friends, then I would suggest finding your own seperate groups of friends as well. While she spends time (in the freezing cold, staring at cars) you and your friends can go out. What is most important to keep in mind, is that, at the end of the day, she is still your girl. At the end of the day, you will still sleep in the same bed, together.
So, to sum it all up, don't worry. Allow your girlfriend to socialize with these guys, after all, if you two spent every second together, I'm sure you both would feel a little stifled. In the meantime, find ways to enjoy yourself while she is with them. Indugle in your hobbies or interests that she doesn't share with you.
Afterall, relationships are about compromise, but those compromises don't have to be dreadfully unbearable
-m