By the time the club was established, Mjolnir Hammer, Dekomori Sanae, was a junior high student. How can she join the club? Was it because her junior high and the senior high that Tyrant Eye, Takanashi Rikka, attends is an escalator school? Is it common for student of an escalator school to attend club of the higher school in Japan?
If we don't count Dekomori as an official member of the club, then the club would have only 4 members (Dark Flame Master, Tyrant Eye, Mori Summer, Tsuyuri Kumin) which is one less than the minimum for a club.
Note: I'm not sure that the school is an escalator school. The uniform is similar and I think Yuuta once said to Dekomori, "Oh, you attend our junior high, huh?" which suggest that the school is an escalator school, but I'm not sure if I remember it correctly.