Start by making eye contact and saying Hi. Then if they’re receptive ask some innocuous questions like “why did you come here” or “what did you think of this talk.”
I find this is the basic of networking hack. And this is what most people do. Say "hi" and ask their opinion or their story.
Or you can offer your opinion "I found that new technique very interesting". "I didn't understand why that's better than X".
Or if you're super focused on something specific go for that. You're looking for a job, a client, an employee, ask for that directly. "Hi, I really enjoyed the talk and I'm looking for a job in the area. Are you working on something similar?".
Ask a specific question. "Excuse me, did you catch what the speaker was saying about async routines? I think I've done async before but I'm not sure." Now the person you're talking to can explain something, and you can segue the conversation into some related topic.
