I guess it depends on how serious you are with these guys. And really, I dislike using the term "stealing" when it comes to people. As if they're property of yours. And I know you probably didn't mean anything by it as it's just a saying, but since people aren't property, how can a friend steal someone from you? When my friends and I go out, every girl is fair game. If you're the kind of guy she is into, great, if not, oh well. I wish the friend who got her number all the luck with her.
Now, if you're meeting guys on your own...had a couple of dates and she comes in trying to flirt and get him to fall for her, that's disrespectful, and maybe you should stop hanging out with her.