If you want to add game to your library just click "Subscribe" button under lobby message, otherwise use command:
/subscribe [GAME]
example:
/subscribe Counter Strike: Global Offensive
If you want to create lobby just ping bot with typing @gamingrootbot (or your newly created instance) to textbox and wait 'till bot automagically give you a clue with your subscribed games! Once you pick a game just click on proper field to create lobby and tag everyone that subscribes same game in case they would want to play with you!
Special thanks to JuniorJPDJ for creating this desription in our native language.