There isn’t a list of covens that I’m aware of. The major covens who do impressive stuff are known on the field. I know of 2 larger Winter covens Highguard, but that’s partly because a couple of my Chapter mates belong to one of them.
There is a mage meeting somewhere once a summit, someone comes round to let you know where and when, be worth sending someone to listen anyway.
As far as creating a coven you can do this off the field, create it online and register everyone you’ve got with the right realm lore before you hit the field. You’ll need your Coven Oath to type in, that joiners swear to when they join. This has the benefit of saving some mana, and a bit of ref time on the field. If not get the coven name and oath registered online and then feel free to make up some ceremony to add your mates and do that on the field. (Write the oath in your notebook then you can intone it slowly and portentously to reassure everyone you haven’t just forgotten it entirely!)
Recruitment is a matter of trying to persuade others to either become magicians and pick Winter as their realm lore, or finding people already with Winter lore and persuading them to join. Come up with some arguments why they should join you instead of others, possibly worth a pamphlet? Do note Covens can only be of one nation so you are only appealing to other Highborn - this may make the Virtue angle easier to sell!
Direct recruiting by facebook is frowned on, although you are welcome to put an intro to your Chapter post on the HG FB page so we know to say hi when we get to the field, and say you are open to additional members on there.