You need to create a room and then invite your friends to the game any way you like.
Your friends get the invitation with the number of the room. They should enter this number in the game and you are ready to start playing.
If you get a number, you need to enter it in a separate window (the tab " With friends").
 
To create a room follow the instruction.
On the main screen, click "Play". Then choose the tab "With friends". Choose the game, the score, the bet, and the number of players. Then click "Create" on the game you have chosen. You can see empty circles with the "x" signs. Click on them and invite your friends with any way you like.