Hello, I'll be helping to lead the ride on the 9th, looking forward to it :-) If you haven't seen it, GCN is usually a good source of advice: https://www.youtube.com/watch?v=kGyTdklpnVM
To third the other suggestions, a couple of decent mid-weight base layers are really versatile, and can extend the use of your summer gear well into autumn and even winter if you layer them. With the recent weather (ie windy but not actually that cold) I have found a windproof layer really important, either a jersey with windproofing (usually expensive) or you can get baselayers with windproof panels on the front which work really well (cheaper and you can wear under anything). Most of my kit is intended for cool-weather spring/autumn riding, for the really cold days I just put it all on at once :-) Cycling being the new golf, you can spend an unlimited amount of money on expensive garments specialized for very a specific set of conditions - but I think basing your clothing around layers is a better solution than spending a lot on eg. a really warm jacket that you only ever use a couple of times a year. If you are really dying to invest in something nice, the Castelli Gabba really is wonderful in wind/rain, but you would still want to layer under it for the cold.
Keeping knees warm - your options are shorts + leg or knee warmers (the versatile option), 3/4 length or full length tights. This year I have got the most use out of shorts + knee warmers, but it has been mild. Look for something made from roubaix fleece, and possibly windproofing. dhb (Wiggle's in house brand) are great value for money in this department.
Don't forget your extremities, I find my body runs quite warm once I am moving but the bits that get really cold are my hands and feet, so I nearly always wear overshoes, and double them up and add thermal gloves on the really cold days. I've recently experimented with oversocks, and been impressed - work well on their own on dry windy days, and as a layer under overshoes on really cold or wetter days.
I have found Planet X a very cheap source of basic but functional staples - base layers, overshoes, etc - probably not the best in terms of breathability but perfectly adequate and great for layering up to add a bit more warmth on cold days.
Specifically for the intro ride, I am going to err on the side of dressing too warm, we will try to keep moving whenever possible but it might be a bit more stop/start than on regular club rides - as the focus is on learning skills and riding as a group, we might stop to talk about things and will always wait to regroup at the top of hills.