Just show up on a Saturday morning around 6:50 at Ashton Way Car Park in Keynsham and make yourself known to a member of the committee, who are easily distinguishable by a yellow stripe on the their club kit. They'll have a brief chat with you and give you an option of groups you would be suited for. Group 1 is the chain gang, Group 2 is a majority of the club (I'm in group 2.5 myself, which is the just moved up from group 3), and group 3 tends to be newer riders, both in general, to the club, or new to group riding. The ride ends at the Keynsham cricket club, where there is coffee/tea/other drinks and a selection of cooked breakfast food for a reasonable price.
If you're on facebook, here's the club page:
https://www.facebook.com/groups/141233032753071/