There's definitely an ugly side to some cambridge drinking/dining societies - sexist, chauvanistic, elitist, exclusive, etc, etc - but for the most part they're a fun and, for many, integral part of the social life here - a chance to go out with a group of friends, meet other people from other colleges, network and so on.
Membership criteria is varied - normally in your first year you'll form your own fresher's drinking society with a group of friends, and so for the most part, anyone is included. However most colleges have college wide societies for older years which often have more exlcusive criteria - anything from popularity, sociability to sporting ability. In my experience though the secretiveness and exclusivity of these societies is over hyped (except maybe for Wyverns or the Pitts club or something like that) and mostly anyone who wants to can be involved.