If I wanted to stop somebody, I would calmly walk right up to him and grab him. If I wanted to talk to someone, I would politely ask, "Can I talk to you for a second?" and lead him away from his friends. I can't remember these tactics ever failing.
Here is some good tactical police advice from, of all places, Mother Jones:
Consider why barking the command "Come here!" doesn't really work. Thompson explains in his article,And it works.
You have just warned the subject that he is in trouble. "Come here" means to you, "Over here, you are under my authority." But to the subject it means, "Go away—quickly!" The words are not tactical for they have provided a warning and possibly precipitated a chase that would not have been necessary had you, instead, walked casually in his direction and once close said, "Excuse me. Could I chat with [you] momentarily?" Notice this question is polite, professional, and calm.