Enra, Benima, and Uane: No matter the amount of interaction they have with the other girls, their ending will not change. These three are antisocial anyhow, so whether they form bonds with the others, ignore them completely, or have a medium amount of interactions with them, they will not change.
Kenchi and Byakura: Their interactions with Kyoko, Kyou, and Izuni, and with Makoto (Kenchi) or Chihaya (Byakura) are CRITCALLY important to getting their true endings. If Kenchi doesn't interact with his siblings either more than half of the time or all the time, they begin to think he treasures his idol job above them and they develop ill feelings towards Kenchi. Kenchi is so depressed that he quits as an idol before his true ending can be reached. If Byakura doesn't interact with Izuni either more than half of the time or all the time, the pressure of being an idol and all the negative things that happen just become too much and she suffers a mental breakdown. Failure to allow Kenchi to interact with Makoto enough throws him into a state of “withdrawal” and he begins almost hallucinating that nearly everyone he sees is his beloved “girlfriend”. Eventually, it becomes so bad that his siblings have to pull him out of the idol world completely. Failure to allow Byakura to interact with Chihaya enough will cause her to start believing that she'll never be as good as Chihaya, and that she's useless. No amount of coaxing from Producer will get her to lighten up, and Byakura either quits (if you would have gotten her perfect ending aside from her interactions with Chihaya) or suffers from a severe panic attack onstage and is forced into retirement (if your treatment of her throughout was just so-so).
Koko, Natsuki, Neku, Chiharu, Guru, Aime and Ruka: To achieve these girl's true endings, they must interact with the others, though the amount they interact with them doesn't matter. As long as they have chances to be around other people, it's alright.
Natsu&Nami, Mariko, Haru, Hiromi, Fuyu, and Soumin: These girls will operate best when interacting with the others half of the time or more. While it isn't CRITICALLY important that they get to interact with the groups constantly, they appreciate spending time with people. If you fail to let them interact with the others enough, they will feel upset and will fail to cheer up enough for a proper final concert.
Hiromi in particular achieves a special true ending if you allow her to interact with Nakamura Kyou as much as possible. Her best true ending involves her marrying Kyou and both of them retiring as idols. Hiromi's true ending can still be achieved without this addition, but she seems her happiest if you do let her see Kyou.
Yori: Yori is the only idol of the group whose true ending can only be achieved by having her interact with others the LEAST amount possible. While Yori is about the same level of antisocial as Uane, Benima, and Enra, she is also incredibly socially awkward, a loner, and even a tad bit shy. Yori is very content with her friends and doesn't really want to get heavily involved with more. She often claims that most of her problems stem from her always having to help everyone she knows with THEIR problems. If you make Yori interact with the others too much, her true ending will be negated by all the people she “bonded” with bombarding her with their problems. Yori breaks down and loses her indifferent composure for (possibly) the first time you've ever known her and admits that she can't stand having to always be strong and take care of other people. She admits that she's just getting way too stressed out and she can't handle having so my problems thrown at her to solve. If you would have gotten her true ending aside from interactions, Producer is given one last chance to calm her down (through a touch interaction). Failure to properly complete this final test (by any means of comforting and meaningful contact, like a brief caress of the face or a prolonged touch on the shoulder) results in Yori realizing that she can't even rely on Producer and reluctantly quitting as an idol. If your treatment of Yori throughout was so-so or just plain bad on top of her interactions being too many, you will not receive your final chance to comfort her and keep her around as an idol and she will just quit, admitting that she's not even sad. Her indifference has extended to her idoldom.
The exceptions are if you limit Yori's time with everyone EXCEPT Kyoko, Izuni, Chihaya, and Takane. If Yori interacts with Kyoko more often than usual, her true ending can still be achieved. This is because Kyoko shares her indifference and will not bombard her with problems, thus activating her little nervous breakdown.
Chihaya and Takane are very mature, and also would not be the types to bombard Yori with problems. Thus, interacting with Chihaya and Takane can actually prove to be good for Yori, and may help cancel the effects of her talking to others too much. If you fear you may have given Yori too much time with others, let her talk to Chihaya or Takane more.
Izuni is training to be a motivational speaker, so some time with her is good for Yori. However, you must be careful with letting these two interact—if Yori talks to Izuni too many times in a row, she'll begin to get very annoyed and you'll be back to where you started.
As for the extra group:
Megumi: Nothing much happens no matter what kind of relationship your idol forms with her, but the more Fuyu talks with her, the more likely she is to achieve a true ending since the two are best friends.
Mia: Like Megumi, no matter what relationships your idol has with her, their ending will not change. Chiharu's true ending is easier to achieve if you let her talk to Mia often.
Akita: No matter the relationship anyone has with her, their ending will not change.
Gin: Gin is very troublesome. If your idol completely ignores her, nothing much happens. If they bond with her, they are more likely to get a true ending. If they have medium interactions with her, however, most of them (the exceptions being Uane, Enra, and Benima) will not be able to achieve a true ending. If Gin is unable to bond with anyone, her OWN bad ending comes into play, and she eventually dies from an accident induced by drunk driving. The idol that spoke with her, but didn't bond with her, will hear this news and become incredibly saddened that she(/he) couldn't do anything to save her. If the idol DOES bond with Gin, she is able to quit smoking and drinking through a long and painful process with the idol's help.
Shiori: Interactions with Shiori don't really matter...unless your idol of choice is either Kenchi or Natsu. Shiori, like Kenchi, has an obsession with Makoto and will stop at NOTHING to destroy everyone who gets in her way. Too many interactions between Kenchi and Shiori may cause Kenchi to enter Dark Kenchi mode more often and could negate his true ending by causing him to fall into his “withdrawal” ending. This can be negated by having him interact with Makoto at every opportunity.
Natsu is less severe, but causing her to be around Shiori too much will cause her own crush on Makoto to surface, and she may become very cross. It won't have a big affect on her, but it may cause problems with her relationships.
Miyuzaki: If your idol of choice is Haru, interactions with Miyuzaki are a good thing, but are not critically important. Other than that, Miyuzaki can be basically ignored without it causing problems.
Kyoko: Kyoko can be ignored without consequence unless your idol of choice is Kenchi. If Kenchi is not allowed to interact with Kyoko enough, she will become cross and becomes so upset that her relationship with him deteriorates and she begins to almost hate him. This upsets Kenchi so much that he quits.
Kyoko is also one of the only idols that can interact with Yori more than half of the times possible without activating her bad ending.
Kyou: Much like Kyoko, interactions with Kyou only matter if your idol of choice is Kenchi. Failing to let Kenchi speak to Kyou enough makes Kyou cross with his little brother, so much that he begins to ignore Kenchi, who gets so upset that he quits.
Also, to achieve Kyou's own true ending, you must let Hiromi interact with him a large amount of times. If you do, Kyou and Hiromi get married.
Izuni: Izuni is a lot like Kyou and Kyoko in that the only time it matters how many times she interacts with others is with Byakura. Failure to let Byakura talk to Izuni enough will result in Byakura's nervous breakdown. Other than Byakura, it doesn't matter.
Having Yori interact with Izuni can help cancel out letting her talk to others too often, since Izuni likes to help people with problems instead of load people with problems. A dose of Izuni's speeches once in a while will help Yori to relax. Too much time with Izuni, however, will negate this and make Yori extremely annoyed.
