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Funny how we help each other on the forum and then stealing each others girl's in real life haha!

exactly! this is what happens when you come from scarcity frame. you see a problem instead of opportunity.
If you find guys who are on a path of improving themselves align with them. Become friends with them. Become friends with some girls. Hang out with people. Build a life for yourself.
Just imagine a scene one year from now, you're in fancy club with your high value male friends, you have girls in your group and another "PUA" approaches your set and tries out some lines. You will not be threaten by him, you will be amused by him. You can even play along...
As for where you're right now and your questions.
When you're with a girl and another guy approches and just starts talking to her. By doing that he is so disrespecting you, and you do not tolarete that. What you do depends on the situation (where are you with that girl, do you have your friends nearby, can you make jealousy plotline...). One thing you must not do is addressing him first(he is the one who should talk to you first). You can just take the girl by her hand and move her out of there, you can cut the guy out with your body positioning...
When you want to hit on a girl who is with a guy, you should respect him, open him, talk to him, talk to her, game them both and what usually happens is the guy realize the girl is into you more than into him and he just leaves.
But this all depends. If you are in the right mindset, you'll know what to do.