"Then, why not we ask Youko-sama to come?"
Mizuno Youko (水野蓉子) -sama was the previous Rosa Cinensis, sister of Sachiko-sama. Sachiko-sama's inclination was uncertain, but if someone must be pulled in no matter what, then nobody is suitable except Youko-sama.

"She always looks busy, though."
Saying that should have nothing wrong, but when Sachiko-sama said "she looks busy," then that line could not be used. What about calling the other two previous Roses except Youko-sama? Not sure why, but something sounded funny.

Next, the faces of Yoshino and Shimako came up. But that would just be a common member gathering, and if Sachiko-sama really wanted so, she should have suggested without beating around the bush, "Why not have a Yamayurikai executive member camp?"

"Then, what about onee-sama's classmates?"
"Yumi, did you hear what I said? I asked you to invite who you want to invite to the summerhouse."
I know. I know every word of it. That is exactly why I am troubled. A desparate moarn cried out from Yumi's heart.

This question was harder tham those in any subject's test at the finals. So difficult that Yumi would as well just want to give up and ask for the solution set.
"Anyway, ask around during the post-exam break. I will ask again at the term-end ceremony."
In the end, this question remains a question for homework.
Even amidst all the summer homework, this question is using the most brainpower -- probably.