Make sure your girlfriend likes all of the guys, or at least can tolerate them. Don’t have a million of your friends over, only have like 2-3. It’s also a plus if your girlfriend brings one of her «girlfriends» with her.
Be one of the guys, but when you’re being one of the guys, don’t act completely like you usually do. Don’t do anything your girlfriend would dislike. Go and do the usual stuff you and the guys do, just don’t completely ignore your girlfriend.
Hang with your girlfriend. The other half of the time is to pay a lot of attention to your girlfriend, like a date. A good idea is everybody goes inside and watches a movie. Then you cuddle with her. Or go somewhere to do something fun, and flirt with her. You could just simply be like one of the guys, but do it with her.
Do not ignore your girlfriend, or your male friend!
How to Be Charming
How to Drive Your Boyfriend Crazy
How to Get Over a Guy Who Doesn't Like You
How to Ask a Guy to Dance