I have been dating this boy for a while now for like 4 or 5 months I don't know, anyway lately my boyfriend has been hanging around my friends with his friends because he friend is dating my friend.

But this new girl joined our group and said to my friend that she liked my boyfriend and would steal him from me. That’s what my friends said, I got even more worried and had heard that she was flirting with him.

I have total trust into my boyfriend it’s not him that bothers me it’s her she is my friend and I feel like flirting with him in front of her to make her jealous but I know I shouldn't.

I have no clue what to do please I need your advice ASAP! Thanks

You are operating on hearsay. You don't know that this girl really said that.
If you trust your boy friend don't worry about it. Somebody is always going to flirt with somebody.
Flirting with someone to make someonelse jealous is childish. Don't do that, You either trust your boyfriend or you don't.

But this girl steals all my boyfriends or likes them I'm just so scared I believe my friends because 4 people heard her say she liked my boyfriend, and would steal him from me. I'm still so scared. What if at school she starts hanging in my group, it would be awkward for me and my boyfriend. Yes my boyfriend does know.

I have no clue what to do!

Relax, do nothing. If your boyfriend knows her game then he will handle it for you. If she has done this in the past and tries it again, she is no friend and if everyone knows it, including your boyfriend, then you just have to stay cool, calm, and collected, and in control of yourself while she makes a fool of herself.