Yeah.  Thanks in advace for the help.

Hover the 'Groups' icon on the menu and go to 'My Groups'. Find the name of the group you want to quit and click on it. On the right, there will be something called a 'Group Menu'. On the bottom right is an icon that says 'Quit Group'. Click on it.

hi! I want to leave one group, but I am playing a team match for this team at the moment. If I will leave this group will I automaticlly lose this game?

no