Go into the group itself, look on the far right of the group options. There is a link there that shows 'Quit Group'. Click on that. You're then given an 'Are you sure' box. Click "yes" if you really do want to leave.
Any team matches you may have from that group will continue till they are done, but if you joined any vote chess from that group, you'll be auto removed from those as soon as you leave the group.
As the topic says, if i want to leave a group how can i do that?