If they're anything like the web dev shop I work in, absolutely not. We're free to grab drinks after work and we're free to go home, there's never any pressure to join.

I assume GitHub is all that and a bag of cookies - you can join and chat and build relationships with people, but under no circumstances is a gun held to your head.

Chances are, if you work there and you enjoy the environment, you'd WANT to go out for drinks with them!