Robotwars gives you a virtual battlefield where you program an AI tank and pit it against your opponents.
To run this for your own meetup, just point people to the github link above, all the instructions are on it. The tricky thing is setting up a server to host the battles on. When we ran this in Christchurch (New Zealand) we also had a server available here however, it's not going to work out of the box. Other meetups have had success allowing people to just commit their tanks to a common repo and then pulling them in.
If you happen to want to be a hero and the infrastructure linked to above, make it all Docker'y 'n stuff that would be awesome. However, as is it's pretty usable
Here's a video of one of our battles.