A general best practice of UI design is to make things work the way the user expects them to work. Seems obvious, but it's actually pretty hard. There are two ways to figure that out: ask them (do user testing) and look at what everyone else does. Poor man's user testing is listening to all the cranky people like me and see where they get stuck.
The big players here: Facebook and Eventbrite. They both have easy "i'm attending"-type links prominently displayed.
The major fail in the current Ops Centre UI is that in order to attend an event, you have to find the choose a side option. But users expect that in order to join an event they should click "join" somewhere. There is no join button. To make matters worse, the "choose a side" button is buried down below, in the third tab on a set of info tabs.
Ideally, the button to join/attend would be on the main page in plain view. Now, it can send the user wherever you want - to a page that makes them first choose a side, or pay, or whatever - but that first action of saying "I want to join" is what they expect.
The most important thing you could do to improve the UI of the site would be to address that issue, IMHO.
Thanks for listening,
Rywalker / H6-2
"Acta non verba"