Ofcourse your jealous, you have feelings towards this person and you are lonesome, and you know that is she off doing stuff with other guys..
As you put it, they are only friends... friends, do.. firends things
I've taken girls to movies and ice skating blah blah even when they did have a boyfriend, your allowed to be friends with people and your allowed to spend time with your friends, jealousy is so common though, everyone experiences it in their life and it is completely normal.
And (correct me if i'm wrong) is she the one who has told you that she is going out to dinner with them and not doing anything? Because think about it, she might not of had to, you sound like you've fallen for her, which is a big thing and obviously you want to be with her and you are very happy with her.. continue to be jealous, it's normal don't let anyone tell you otherwise... you are fretting to be with her pretty much, seeing her that often would suck balls, but hey.. if that's how things have to be then hey ! that's how they are
Try spending some time with your female friends, help take your mind off things, unless you can't control yourself lol..
But hey bud, she's all good mate.. just relax and let things flow their own way, it's no real biggie, friends are friends