Beta Readers are some of the people to whom I send early versions of my books for feedback. Usually, these are different from Alpha Readers, who include industry professionals like my editor, my agent, and my writing group. Beta Readers, instead, are usually fans and ‘average’ readers, used as a test audience. I don’t expect them to offer solutions to problems; more, these are the people I want to use to gauge how the book will be received.
Most of these people fall into two groups. The first are old friends who have been reading my writing for a long time, and whose opinion I trust. The second are people who have made insightful comments on places like the Seventeenth Shard, Tor.com, or my facebook page. They are generally people well known in the fandom community surrounding my books–people who have good reputations, with whom we feel we can entrust early copies of books without leaking them.
We do pick from general fans sometimes to do beta reads, but there are a LOT of people who want to do this–and not many slots available. Usually, we pick people who have a special expertise relating to a book I’m working. (We might pick a person who has been an EMT, for example, when reading Stormlight–to help with Kaladin’s surgery scenes.)
I don’t generally pick Beta Readers myself. I leave this to my team, mostly Peter Ahlstrom. I suggest not pestering him with requests, however. Instead, if you really want to beta read, participate in the fan community and get to be known there. Another great way to help is to find typos that HAVEN’T YET been found and post them on the appropriate thread for that book on the Seventeenth Shard. (Don’t just send these vial email; chances are, Peter already knows about them and has fixed them in a newer edition of the book.)