Besides the obvious roles of getting him drunk, dragging him into strip clubs and filling his head with inane commentary from the Simpsons or whatever piece of puerile claptrap Adam Sandler is shilling this week, a man's buddies are ostensibly there to watch out for his best interests: which might prove to be a problem.
You're new on the scene. They're going to be (justifyably as it turns out) suspicious of you and your intentions. They might begin to notice his descent into your clutches and attempt a rear-guard action to prise him away from your grasp. And here's the problem. He has years of experience with his friends, he knows and (except for that time with the donkey and the handcuffs) trusts them completely. So whatever they say to him he is going to take very seriously.
But you have the greatest weapon of all. His libido. Just remember: simply listening to that friend of his gives you a headache and gets you out of the mood, right? It's impossible for you to feel intimate with him for at least a day after being exposed to his unwanted comrade, isn't it.
Just make sure that the night after you've been huffy and withdrawn you put on a hell of a pyrotechnic display in the bedroom and apologise for the night before during the lull after the storm. You know, that time when the endorphins flooding his body still prevent normal brain function. You're sorry you were upset, you try to get on with him, but he (1) said something sleazy to you once as a joke, but it really wasn't (2) looks at your boobs when your boyfriend isn't looking (3) treats his own girlfriend unaffectionately (4) heck, you can be creative can't you? It's all about building doubt in your man's mind about the quality of his friendship with your rival.
Now here's the rub. You don't want him to have no buddies at all, you just want him to have the right ones. Heaven knows, once whispers of being a 'cold bitch' begin to circulate that kind of image is hard to defrost. Particularly if there is a teensy pinch of truth to it (Hey, don't be offended if you are. It's a strength. Really it is). So, be super friendly and garrulous with the friends (and girlfriends of friends) that you approve of. Suggest intimate dinner parties, go to concerts with them, even go camping and the such. Do the kind of things that couples do, and as each day goes past you will gradually distance your man from the undesirables in his circle.