I thought Sumireko created the Secret Sealing Club with the intention that no one would join.
She's introverted, and would rather be left alone most of the time.
This kinda seems out of character for her to just suddenly invite members.
She created it to avoid making friends with her "inferior" peers, but inviting random Gensokyans, whom she's fascinated by, to become her students and show off how amazing she (thinks she) is definitely feels in line with her usual egotistical behavior.
She did make friends with Mokou, after all, so it's not like she's incapable of getting along with other people altogether.
Huh~
So this is what those rumours have been about...Recruiting now!!Great fun!Secret Sealing ClubPSYCHIC POWERSAnd the training's supposed to be real easy too, yeah.Really?You mean I could do it too?These mysterious figures... What do they intend!?Want to use them?
Lift the heaviest masses with Psychokinesis!The rice-dealer's boy apparently managed to make a huuuge bag of rice float in midair, I heard.So join our club!Whoa~ I wanna see that, that sounds cool as heck.Discover your partner's infidelity with Clairvoyance!!