When you log into your steam community panel, you will see your Groups tab. Find your group (you need to be an administrator) and access the admin page by clicking Visit admin page. You will see the first field which is ID copy that number and place it somewhere you can access later. This is your Steam Community ID.

In your server.cfg you simply add:
sv_steamgroup 813123

If you wish your server to be exclusive to your steam group, you can also set
sv_steamgroup_exclusive 1 

This will only allow players from your steam group to connect directly to your server, before allowing anyone else to join.