They're done alone as well, even spouses are split into separate studies, so you're less likely to spot problems.
To be fair this isn't really true. Studies with couples together, or even whole families are encouraged. There would have to be a special circumstance to study with spouses separately.
You are right however that they wouldn't want group discussions like Alpha. The whole format is based upon controlling the environment and the questions and answers. Sure, people are allowed to ask questions outside of the material, but it isn't encouraged.