Public groups will be visible to everyone one on the network however private ones can be used with some authorizations. I found this quote from their website which might help you to understand further.

Mark joins a public group about photography and creates a private one for local photographers in his area.
Source

So there is possibility to create a private group which will only be used for certain set of your local people within a circle.

Regarding niche community, you got it right. There are different categories with different fields. Like Photography, Gaming, Health and Fitness and Crypto to name a few.