How to attract people to attend newly established alumni event

I am orginizing an alumni event. We used to have several event but was not so successful. We now have over 100 alumni in the city of Shanghai, half of them are 30-40, half of them are over 50. How can we plan an event that can bang the group and attract many people? Anyone could give me suggestions? Thanks

Answer this question first: Why?
Why would the alumni of your particular organization want to hangout with each other?
Assume that they are very busy with their lives, have extensive social, professional and personal responsibilities. Why would they want to come and hand out with fellow alumni?

Once you have answered and verified the answer to this question -- then you will be able to design a strategy that will attract people -- by meeting their need. (Verfiy means that you have "doorknocked" that group members and confirmed which activities/events/needs/opportunities they will invest time in making happen)

Think of the event as your product. Your organization is the start-up. What is the core value proposition of the product? And then how will it be delivered, and how will attract "customers" will have a context that has a much greater chance of being viable and sustainable.

If they range from 30-40 mostly i think some kind of Food and Drink event (with alcohol) would be good. Also since it's a networking event a good thing to include are some details about where people work and what what they do there. I would recommend on focusing on making connections with people at other companies. The event should have an attraction, but it shouldn't be distracting. A nice location is enough to get people to come. However the event should consider some highlights. Try and bring a speaker or two to discuss networking skills. Then let everyone walk around and do some of the networking stuff they learned. That way instead of just meeting old friends people can meet new friends. Also since there is a broad age range a more mature event would benefit everyone.

Start your event with something attractive cultural activities like dancing and others later start your event and finally again end your event with attractive activities.

Even if your event is boring to users also this can help you in getting more users.