"Bei" is used in german when we want to indicate that we live/work etc "under" someone, for example "under someone's roof". As a result, "wohnen mit+Dativ" means "I live with someone in the same house", but the house is not necessarily his (i.e. the second person's). Otherwise, when someone lives in his parents' house, "er wohnt BEI seinen Eltern". The same applies to work. "Ich arbeite mit John", means that you work together in the same company/project etc. But "Ich arbeite bei John" means that you work in John's company. You also use "bei" when you want to name the company, for example "Ich arbeite BEI Siemens".