"I've worked at Google (for three years)." We use 'at' for companies/places of employment. "I've worked at Siemens for ages." "He's worked at that company for 2 years now." HOWEVER using Present Perfect (have worked) in this context means you still work there, so you need to add how long you've worked there. Otherwise you'd say "I worked AT Google before I got my present job." (using Past Simple). "I've worked IN Google" is incorrect (unless it's a specialised computer term I'm unaware of). "I've worked WITH Google." means we have cooperated on a project. Using Present Perfect here means that this happened in the past. (I've worked with Google, Siemens and Braun, but never with The Bank of Scotland.) (This has happened in my life so far). Be careful of not using Present Perfect when you actually want to express something in Past Simple. (That's a very long series of lessons with a teacher). I hope that helped.