Probably Huntik, an American-Italian cartoon aired between 2009 and 2012.
- The blonde guy, Lok, goes to school in Italy;
- Sophie (the skirt girl) is a student from the same school;
- The middle-aged, red-haired guy is Dante, one of the Seekers; Seekers can summon creatures known as the Titans, and Dante particularly can summon Caliban, as he does in the first episode (see video below);
- the second teen girl (dark-haired one on the right) is called Zhalia, she joins them later in the series.
Here's a sheet with all Titans and their summoning amulets:
Throughout the series, the kids fight against the evil Organization, whose goal is to rule over the world (such originality !). The Organization seeks a huge, legendary-like Titan to ensure their domination. So basically, they're like villainous Pokémon teams, but without the catchy name.
Plagiarising my own answers to Animated show with character who has a stone that summons creatures and Fantasy cartoon show aired in India around 2005-12; main character searches for his father. Involved magic rings