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.

Pgj1997 said:

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.