Therefore, a = 4；;
(2) As can be seen from the table, two people have done it six times.
Complete the statistical chart as shown in the figure;
(3)∵ There are 10 members in the group, and 3 members participated in the activity of 10.
∴P=3 10，
Answer: Choose a member from the group to report volunteer activities to the school. The probability that a member who has participated in the 10 activity is selected is 3 10.