I try not to approach them when they are having a conversation with another girl.
I usually wait for them to be alone or when their conversation ends.
As a rule never ever approach a girl when she is with another client! This happened to me when I was talking to a girl and some douche bag approached her. I was like WTF in my head, little did the guy know that she was my regular and she just brushed him away. But even the girl was like WTF, major sign of disrespect when you do that.
If you want to talk to a girl when she's with a guy or talking to another girl just try to make eye contact with her and she'll get the message.
Well, if I'm a guy, first of all, I would like to say hi to her, if she talk to me, then that's the time I approach her and drink beer together with her.