Search for the player you want to duel, press 'Ctrl' and then click with right button in the player and select 'Duel', then select the number of players and the amount of Pokémon.
In case of team duels, you must first invite your team, after that invite the opposing team.
To accept a duel just press 'Ctrl' and click right button on who made the call and select 'Accept Duel'.