A druid's grove and a circle aren't the same thing, to be fair. What we see is a druid's grove. A sacred space inhabited by a small community of druids who seem to live there full time.
A Circle isn't bound to a specific group and druids who are a part of the same Circle don't necessarily live together. There are many religions within any given Circle as they stretch beyond the limits of political borders and tend to not have a single location where they operate. Druids, by and large, roam. They don't have big churches or temples.
The grove we visit seems to be a sort of natural temple to Sylvanus specifically, which would explain why all the druids there worship her above all others. They aren't likely all part of the same Circle, they just share the same faith in Sylvanus and decided to dedicate their lives to her and her grove in the same way a cleric might choose to dedicate herself to a specific temple.
