Honestly don’t blame her. Doesn’t help that Ijichi didn’t let anyone else know to see if they were okay with it. Also, sure it was polite for the guys to ask if they can come along but maybe don’t buy into other people’s plans. Maybe I’m just over reacting
I don't agree with not buying into other people's plans, because it can be often common way to hang out, especially when larger group of people like most of class is already friendly with each other, and such "buying in" can led to people becoming better friends or striking out new friendships, like Ijichi intended. It's just she should absolutely let others know and consult them before agreeing, especially if there were people like Amane who rarely hang out with others and barely know anyone.
In case of something like the cast of "You and I are Polar Opposites", I can totally see characters from MCs' class wanting joining any activity started by few of them, but the vibes there are very different. I even think something like that ("Hey, what you guys doing? Can we join?) happened there once or few times.
Extroverts terrify me. Who in their right mind would invite so many people without consulting anyone else they had previously made plans with? I can't fathom how anyone would be okay with that sudden development.
I think Ijichi treated it like the initial period and season of "integration activities", like those that schools or companies sometimes makes you join when you're new student/worker, with her (incorrect) implicit understanding that everyone is still trying out to get to know as many people as possibly to forge new friendships for the high school period. It was still only two weeks into beginning of high school.
By my own experience Amane should have just left after all no real point in sticking around if the original plan is already out the window
That could just make it worse, as other people would then be curious or worried about why she left so early, which could spiral into some pointless drama. She still barely knows anyone, and does want to have some friends eventually, any risk of making it a bigger thing than just between her and Ijichi would be wrong move IMHO.