About a dozen people showed up for the impromptu cat tour, led by John Edwards, who organizes the yearly event, seven years ago. Over a two-mile walk, the group stopped to see some 20 cats peeking through their windows.
Now, hundreds of people attend.
Can't you write a sentence? The phrase "seven years ago" should be at the beginning, modifying "people showed up", not at the end, modifying "who organizes the event"