9. (5 points) Among the students participating in the gymnastics, martial arts, piano, and calligraphy interest groups, each student can participate in at most two interest groups. To ensure that there are no fewer than 6 students with exactly the same selection of interest groups, the number of students participating in the groups is at least $\qquad$ people.
【Answer】Solution: There are 6 cases for participating in 2 activities: (gymnastics, martial arts), (gymnastics, piano), (gymnastics, calligraphy), (martial arts, piano), (martial arts, calligraphy), (piano, calligraphy), and there are 4 cases for participating in 1 activity. Here, these 10 cases can be considered as 10 drawers,
$$
10 \times 5+1=51 \text { (people) }
$$

Answer: The number of students participating in the groups is at least 51;
Therefore, the answer is: 51.