How to Set a Clan Tag in Counter Strike: Global Offensive - CS GO
1.) In-Game click Help & Options.
http://files.fileb.ag/i/00001/kf3mcgwbhq96.jpg
2.) Click Game Settings.
http://files.fileb.ag/i/00001/809rfytidv95.jpg
3.) Next to Team Tag [From Steam Groups], scroll over to your desired tag.
http://files.fileb.ag/i/00001/kf3mcgwbhq96.jpg
4.) Click back or escape to get back to playing.
DONE !!