I have to agree with bad wolf. If they try to make you go to church knowing you're an atheist, that's the same thing as you trying to make them not go to church knowing they're christians. If they try to convert you, it's the same as you trying to make them lose faith. There's no reason you have to pretend to be someone else or be friendly just to make them comfortable when you're not.
What I usually do is, if asked, I'll say I'm not a believer then if challenged, I'll challenge them. If it's just curious questions I don't really mind, sometimes I'll elaborate and that may offend some thin skinned people, but I've never had any problems, maybe because I know so many atheists. But I have been seriously challenged about my lack of belief before, and I didn't pull any punches, fair's fair. You'd be surprised how many people don't mind defending their faith (and you'd be surprised how they just will not reach the same conclusion you reach), so if you feel like they're pushing you into a corner you can fire back. Many are willing to just say, well, have to agree to disagree. They don't really stop being your friends, you just know each other a little better now.
What I usually do is, if asked, I'll say I'm not a believer then if challenged, I'll challenge them. If it's just curious questions I don't really mind, sometimes I'll elaborate and that may offend some thin skinned people, but I've never had any problems, maybe because I know so many atheists. But I have been seriously challenged about my lack of belief before, and I didn't pull any punches, fair's fair. You'd be surprised how many people don't mind defending their faith (and you'd be surprised how they just will not reach the same conclusion you reach), so if you feel like they're pushing you into a corner you can fire back. Many are willing to just say, well, have to agree to disagree. They don't really stop being your friends, you just know each other a little better now.