There may be cases you miss the pop-up invite or it was blocked by your browser. Here's a quick and easy way to enter the game through CS:GO Console.
1. Launch CS:GO, go to [Game Settings] and [Enable Developer Console (~)]
2. Enter[~] on your keyboard and a window will show up
3. Input the command [connect xxx.xxx.xxx.xx:xxxxx] (space after connect, no [ ]) and hit enter. The system will then start connecting you to the designated server.