It seems easy. Bei = by. While this is sometimes the case, it isn't always true, so be careful.

"Ich arbeite bei Siemens": Don't say "I work by Siemens." In this case, "by" means "near" or "in der Nähe von." Instead, use the preposition at or for and say "I work at Siemens" or "I work for Siemens."

"Er ist beim Sascha": When talking about visiting someone's home, don't use "by." Instead, use at along with the possessive. "He is at Sascha's" or "He is at Sascha's apartment/house/place." "He is by Sascha" means that he is standing or sitting next to Sascha.