Translating BEI into English
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.
Prepositions are one of the hardest parts of learning English. Here are some more lessons to help you avoid making common mistakes when using prepositions:
More English Lessons for Germans