I agree with trying to schedule events. It is as simple as a happy hour. Back when I was in med school, I formed a fb group about going to a particular place on Fridays for happy hour and invited people to the group. That was back in '06. I'm going to get together with a lot of those folks in May as one of them is getting married.
The way to approach it is this: Find one or two people who you think might be into that sort of thing. Then, you send out a message to others and say "So and so and I are going to happy hour if you want to join". That way, people understand it is a group thing and that they won't be stuck somewhere just with someone else. That seems to give a lot of people an amount of anxiety (possibly being stuck with just one other person). However, large groups also give people anxiety. If you can project that it is a small group activity, you are probably more likely to get people to come hang out.
Also, try to center it around something like a sporting event for instance. People like the idea of that even if they are not rooting for a team or sport since watching something like that can take away the anxiety that some people have about having to be engaged in conversation constantly. After a few times out, you'll hopefully start to find a core group of people who like to hang out with each other.
