Sting does not join Fairy Tail. Although there may be a brief moment where Sting and his teammates eye the new Team Fairy Tail as they enter the Domus Flau, it does not indicate that Sting has any intention of joining Fairy Tail. In fact, Sting is a member of the rival guild, Sabertooth.
Throughout the series, Sting has been portrayed as a confident and somewhat arrogant character. He takes pride in being a member of Sabertooth, which is known for its strength and competitive nature. Sting’s smug smile when Erza Scarlet confronts Team Sabertooth suggests that he is enjoying the tension and rivalry between their guilds.
Sting’s loyalty lies with Sabertooth, and he has shown no inclination or desire to join Fairy Tail. In fact, Fairy Tail and Sabertooth have a long history of rivalry and animosity, making it highly unlikely for Sting to switch sides.
It is important to note that joining a guild in the Fairy Tail series is not a decision to be taken lightly. Guilds are like family, and members form strong bonds and connections with their fellow guildmates. Sting has already formed a strong bond with his Sabertooth guildmates, particularly with Rogue Cheney, his partner in the Twin Dragons of Sabertooth.
Furthermore, Sting has his own goals and aspirations as a Dragon Slayer. He aims to become the strongest Dragon Slayer and surpass his former teacher, the Dragon Grandeeney. These goals are deeply ingrained in his character and would not easily be swayed by the allure of joining Fairy Tail.
Sting does not join Fairy Tail. His allegiance and loyalty lie with Sabertooth, and he has his own personal goals to pursue. While there may be moments of rivalry and tension between Sting and Fairy Tail, it does not indicate any intention of him joining the guild.