A part of attracting and keeping women is being fully confident that no guy can replace you, while knowing deep down inside that you can replace the girl you're with.
If you show your gf that you don't like her being around other guys, you will come off as insecure in her book, and will convince her that she can do better then you. Say she has a friend, lets call him Joe.
1.) If you ever let her know that you don't want her being near Joe, it lets her know that you are intimidated by Joe, and that you think that he is better then you. Your value decreases in her eyes, whereas Joe's value increases.
2.) If she knows you don't like her being around Joe, the first time you two have a fight, guess who she will go to? You guessed it, Joe. Why? Because she knows that's what will hurt you the most.
Because of those 2 above reasons, you can never, ever let a girl think that you think some other guy can take her from you. But that doesn't mean you should be blind. There are some signals to look for that will tell you whether you can trust your girl around other guys or not. They include:
-how flirty she is with her friends
-how much ec she shares with her friends during normal convo
-does she ignore you when they are around
-does she seem hesitant to show affection to you around them
-how flirty they are with her
If you see these, walk away and find someone else. Otherwise, relax, and enjoy life.