Do adult humans exchange beneficial bacteria? If so, how?
Nonpathological bacteria Staphylococcus epidermidis on the human skin are part of normal bacterial flora that help to protect against pathogenic bacteria, such as Staphylococcus aureus. You can get "infected" by Staphylococcus epidermidis the same way as by other skin bacteria and you get a potential benefit from them.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2807625/
You can get beneficial effects from pathological bacteria passed from someone, if you you develop antibodies to them, so you can become more or less resistant to them. For example, if you are from US or Europe and travel to South Asia, you at high risk to get food poisoning from either bacteria or viruses, bot local people are at much lower risk, because they have "shared" microbes one from another. 
You can get beneficial effects from probiotic supplements (live nonpathological bacteria), which prevent overgrowth of pathological bacteria in your large intestine. The bacteria in probiotics are similar than those in the human gut.