If you go to a large enough group ride in the winter you will see a full range of clothing decisions. There will be the guys wearing hardly anything and will be shivering on their bikes, and there will be the guys wearing every piece of kit they own, along with some hiking gear for good measure. So, what should you wear? Well, like anything else it depends on the situation.
Below 60 you can wear arm warmers.
Below 50 you can wear knee warmers.
Below 45 you can wear your vest.
Below 40 you can wear LS jersey + vest.
Below 35 you can wear your bib knickers
Specific conditions change everything, such as ride length, overcast vs sunny, how you feel, if you're a generally warm or cold person, or if you simply hate/enjoy being cold. For racing knock each one down 10 degrees. So arm warmers below 50, etc.
LONG RIDE: EARLY COLD START (28-34F), SUNNY FORECAST (42F END)
On days like this layers are key. When the sun finally comes up it'll be warm, especially if there aren't too many trees on the route. I like to be cold when I leave in the morning on these rides. Usually I would do Arm/Knee Warmers/Bibs/Jersey/Vest/Thick Gloves/Hat/Shoe Covers. Or Bib Knickers, Arm Warmers, Jersey, Vest, Heavy Gloves with light ones to swap. Basically a mix of kit that is warm but with some elements that I can remove to cool off later.
REALLY COLD (LESS THAN 28)
Just wear everything. LS Jersey, Vest, Toe Covers, Shoe Covers, Wool Socks, Heavy Gloves, Heavy Hat with ear covers, Bib Knickers.
FYI bib knickers are WARM and excellent if you plan to train outside a lot throughout the winter.
This is the trickiest one, because you want to be as light and flexible as possible, and definitely don't want back pockets full of clothing you're not using. You generally want to start cold, and try and dress with just enough to keep you comfortable at the end of the race. You don't want to be shivering, but you definitely don't want to be sweating.
A great way to know what to wear at a race is see what the Pros and Cat 1/2 guys are wearing. Just kind of look around and dress similarly to everyone else.