well... you can kind of check on when someone joined up by mousing over their name... at the bottom of your screen, you'll see a bunch of numbers and letters show up, ending in a = "#####", where #### is literally their membership number on perb... for example, if your number is 2084, then there were 2083 members prior to you joining... and you are member #2084.
It isn't a timeline, but it is an interesting little thing to check on... works for most websites that I'm aware of, anyways...
Another example.... Guardian Angel, your # is 20,447
Mine is.... 833
See what I'm sayin'?
The interesting thing is to find out when the website experienced the greatest growth... which would be found by looking at the join date and correllating the largest change in the perb #'s over a particular period...like a 1 month span.