I've been out with Mitre for a few of their social rides and have been blown away by how friendly everyone is. I let everyone know that I'm a newbie so I've had loads of great advice from everyone.
I thought that, because they're a serious cycling club for racers, the people might be a bit 'cool'. But not at all. Everyone has been really friendly, very welcoming and incredibly considerate too - like making sure to watch out for new members and making sure everyone has enough food and water.
The social Saturday rides are amazing. It's a thousand times nicer to roll out with 20-30 other cyclists and charge around the countryside than it is to head out alone (I tend to get lost). There is plenty of opportunity to chat as you explore Sussex. The rides can be challenging though - often the more experienced riders pick up the pace towards the end, and considering that the end may be 40 miles after you started it can be tricky to keep up!
Providing you're comfortable doing that kind of distance, I highly recommend you give it a go. Don't worry about not being fast enough (providing you're not completely hopeless). But be prepared and take plenty of food and drink. I wrote a short blog post about it:
http://wordsby.me/2010/05/19/learning-to-ride-a-road-bike-long-distances/
Oh, and here's Brighton Mitre's website: http://brightonmitre.typepad.com/weblog/