Well, in some versions, it might start with Team 7 being shocked at first. They may initially doubt Naruto's ability to be in ANBU because they still see him as the goofy Naruto they've always known. But as they see him in action, they start to respect his new role more. Maybe Naruto has to keep some secrets from them initially which causes some tension within the team, but eventually, they come to accept and support his position in ANBU.
In such a fanfiction, the story could also progress with the other members of Team 7 trying to get into ANBU themselves to be closer to Naruto. Sakura might study harder to improve her medical and combat skills, and Sasuke could focus on his Sharingan training even more. They see Naruto's new status as a challenge and an inspiration. Meanwhile, Naruto has to deal with the politics and secrecy within ANBU while still maintaining his relationships with Team 7.
Well, Sasuke might initially be a bit shocked but then he'd try to hide his concern. Sakura would be very empathetic and might immediately want to do something to help Naruto. Kakashi, being the experienced one, would take a more calm and strategic approach to deal with Naruto's situation and protect him from further abuse.
Naruto's reaction could be complex. On one hand, he would be happy to find someone else from his clan. He has always been proud of his Uzumaki heritage despite not knowing much about it at first. But on the other hand, he might be a bit cautious. Karin has been associated with some not - so - good groups in the past. However, overall, his positive nature would probably lead him to approach Karin with an open mind and try to build a connection with her based on their shared Uzumaki roots.
At first, Naruto might be angry. He's put in a lot of effort to get into the ANBU and prove himself. So being forced out would feel like a slap in the face. He might go on a rant about how unfair it is.
Sasuke might be initially shocked. He has always seen Naruto as a rival and suddenly finding out Naruto is a girl could completely change his perception. He may start to view Naruto in a more protective way, as his traditional views of gender roles might make him feel the need to safeguard her now that he knows she's female.
Well, Kakashi is a complex character. When he discovers Naruto is abused in the fanfiction, he may first feel a deep sense of guilt for not noticing earlier. He'd then use his skills and influence in the village. He could start by reporting it to the proper authorities in Konoha, like the Hokage. And at the same time, he'd be there for Naruto, providing emotional support, talking to him, and helping him through the trauma. Kakashi has his own past traumas, so he'd understand what Naruto is going through on some level.
She would likely be devastated. Kushina has a deep connection to Naruto as his mother. She might first go into denial, unable to accept that her son is gone. Then, she would probably be filled with intense grief and anger. She could potentially lash out at those she blames for his death or just break down completely, mourning all the things Naruto could have been and all the time they missed when he was growing up.
Naruto's adaptation to the Anbu in this fanfiction could be a complex process. At first, he'd probably struggle with the strict discipline and the need for secrecy. But his previous experiences in tough situations would come in handy. He'd gradually understand the importance of stealth and how to use his skills in a more covert way. He might also learn from the more experienced Anbu members, and his own determination would drive him to become a valuable part of the Anbu.
Sasuke might be a bit jealous at first. He's always been competitive with Naruto, and seeing Naruto in a special and elite unit like ANBU could trigger that jealousy. He might try to prove that he's just as good or better. Sakura, on the other hand, would probably be worried. She cares a lot about Naruto and ANBU missions are often dangerous.
She might be overjoyed. After all, Naruto is her son. She could immediately rush to find him and embrace him tightly, tears of happiness streaming down her face.
Well, his relationship would become quite strained. Sasuke might feel betrayed as Naruto was an important part of their dynamic. Sakura would be confused and sad. She had come to rely on Naruto in many ways. Kakashi, as the sensei, would probably feel like he failed Naruto in some way. Naruto's departure would leave a big gap in the team, and there would be a lot of unspoken emotions.