I am in a writing group now, as I mentioned in a previous blog posting. It is small, but The Carrot (the sponsor) is helping us to do more promotion and perhaps flush out the writers in the area. There is a Facebook page as well (which I am mentioning simply to promote it here).
What are the benefits of writers' groups? As the Haliburton Highlands Writers' and Editors' Network say in their website, writers circles allow people to "share knowledge and resources"--people with a common interest gather together socially, true, but members can get feedback (formal or informal) about specific pieces of writing, and they can seek advice or make contacts related to professional concerns (marketing, that is).
Our group meets at and is sponsored by an arts collective that runs a community cafe (The Carrot). I certainly have been writing more than I did before I starting meeting with the group. Sometimes writers circles are ephemeral, but some of them last a long time or become somewhat famous, such as the (sort of writers group) Bloomsbury Group and the Algonquin Round Table.