Many Pagans operate within covens or other small groups. Such groups can be intimate. But are they 'traditional'? I suppose it depends on what part of paleopaganism you look at.
Yeah, the Celts had Druids. They had their own training and such. We don't know much about how that worked out. But after training, they were out in the communities 'Druiding' - doing legal stuff, mediating between the people and the Gods, teaching history and other lore, etc. The villagers would have their own tales, shared by elders. They'd do their own folk magic, interpret drops of cream in water, etc.
No small group structure - everything was pretty much village wide or individual. No 'cities', no group cultus.
Here we have cities, and with cities we can have Temples for different Gods and Goddesses. But even with different temples ... people poured their libations to Hestia in their home, and the entire city went out to the temple for the high day of the specific God/dess.
Again, no small group structure. Not even a 'coven full' of priestesses at the average temple.
Were there some groups? Yes. Lupercalia had some group in charge of the sacrifice, and they weren't 'full time' priests (some were politicians as I recall). And the Elusinian Mysteries were not done by everyone, but the experience was totally individual.
Were there some folks who did magic outside of the bounds of the Druids/temples/dominant paradigm? Sure. The Greek and the Celts did not look positively upon them. They were VERY much on their own, no structure, no group worship - they were probably magic only, not a different religion.
WHERE DID THE COVEN IDEA COME FROM?
As near as I can tell ... it was invented by Christianity. Maybe they got 13 members as a 'bad reflection' of the Last Supper, just like the Black Mass is a 'bad reflection' of the Mass. I don't see it anywhere before being mentioned in the Malleus Malificarum, tho' I admit I've not looked that hard. Certainly that's where the phrase "You cannot be a witch alone" comes from, which so many witches/pagans take as 'law' now.
So nothing. Just a train of thought about what the ancients did vs what we look to do today. They did it all alone or with the entire area. We focus on small groups (okay, except for ADF which was formed specifically to provide entire area public ritual ... and even their groves tend to 'choose' who gets to be a full member of the grove, tho' not the denomination).