Attendance analysis for clubs works differently than for normal lesson registers because clubs do not feed into your statutory roll call marks. As such, you won't be able to report on club attendance in the same way (such as through the Students > Attendance > Statistics page). Instead, you'll need to collect your attendance data and calculate the percentage outside of Arbor.
First, make sure the club has been set up to take attendance, and that attendance marks have been input - you can see how to do this here: Taking attendance for a Club
Next, go to School > Activities > Clubs. Select the club, then go to Club Daily Summary from the left-hand menu. This will show you how many students were signed up, and you can count up how many students were present.
You can then do the following calculation: (Students Present divided by Total Students expected) x 100
In this example, 4 students were signed up, 3 were present and 1 was absent, so 3 ÷ 4 = 0.75
0.75 x 100 = 75% of students were present for this session
You could do this for multiple sessions over a week and then calculate an average for that week.