To join the group you must post an application.
Go to Discussions then to the right of the screen choose the application forum where can post without being a member. Please note that spamming the forum will get you banned from the group.

You must mention a couple of things in your applications for it to be accepted.
You must mention your current CS:GO match making rank, hours played at the time of posting your application and a little bit about yourself because we care.

Goodluck!