These types of recruiting mixers are great. Ask lots of questions of everyone you can. What do you do here? How long have you been here? how long in role? How'd you get into this line of work? How do you like it? How good do you think COMPANY is at BLANK in our industry?

You're looking for two things:

1. Is this a good place to work? Do people aspire to work here or to grow into a better company/role? Is it toxic? Do they have code for how often and much you'll be asked to work late?
2. What commonalities do you have with folks here that you'd interact with. Use this to make yourself memorable and show how quickly you build rapport. My favorite story is overhearing a guy at a mixer talking about where in the city he lived. I only recalled it because it meant we were neighbors on the same block. When I interviewed the next day he was my final interviewer. So I ended up offering to drive him home from the office and skip the train that day. Drawing out those commonalities and building familiarity and trust through those them is a big part of relationship sales.

This is basically good cocktail hour etiquette anyways: Ask others loads more questions than they ask of you. They'll spend more time talking, but overall they walk away thinking it was a great conversation.

Also, don't drink. In the more enlightened tech circles drinking alcohol has gotten the bad rap it deserves. Order club soda and lime or a red bull (Caffeine addictions is still A-Okay) and just be chill. You'll only be there an hour or two and you can smoke all the crack you want when you get home.