Wordpress theme for social event/calendar management?

I'm looking for a Wordpress theme or plugin that allows social event management.

Ideally I want to use this for game event organisation. One of my friends runs a large EU gaming community, and i'd like to be able to do the following:

• Social sign-in for members
• Members able to create events
• Members able to sign up for events

Does anyone know of a plugin or theme which allows this It'd be great if there were leaderboards too