Here’s a Guardian article giving you a few pluses of going to a homebrew meetup so I don’t have to do the sales thing too much here.
Most groups involve tasting each other’s beer and giving some feedback about the quality of the brew. You go to learn, socialise and have a bit of fun. Local groups should also organise a crawls, trips to breweries, festivals etc.
More links to National Homebrew Clubs here.
Homebrew Brighton meets up once a month on the first Tuesday in a different pub. See FB, Twitter and the mailing list for more information.
Portsmouth Home Brew Club meet up once a month in different pubs.
Kent Amateur Brewers meet up 1st Weds of the month in The Bottle Shop in Canterbury.