Author groups, I'd say wait for the response here!
If you mean writing groups, then it depends what you want and where you are. Being in a writing group is invaluable to me, having regular contact with other genre writers is great for ideas, for morale, and for getting honest and high quality criticism of your work in progress.
My first short story pro sale came from a throwaway 'I bet you can't write a story about...' in the bar after one meeting. The second challenge like that also sold.
I'm in two groups:
- The T Party (
The T Party - Home), a monthly genre group in London, traditional style crit group.
- MillionMonkeys (
A Million Monkeys - About) , a weekly writer's productivity session (i.e. we meet just to write, to help build up our weekly word count, also London based.
The T Party is open to writers who have publishing credits. John I think knows one or two members.
MillionMonkeys is free, and open to anyone who wants to turn up and write.
There are other very good groups around the country, but if there's nothing organised where you are, then I would start your own. It will take time to build, you need to keep at it and pull people in and you will get something well worth being a part of.