Translation:My friend is working at a clock company.
To be more precise and accurate, you say ありがとうございます to those who are still doing something for you. You say ありがとうございました to those who had completed their (good) deed for you.
Same thing with お疲れ様です (おつかれさまです) "thank you for your hard work", you say that to someone who's still doing the job and you appreciate the work s/he's doing; for example, you say this to a co-worker who has to stay overtime when you're about to depart home from the office. You say お疲れ様でした (おつかれさまでした) "thank you for your hard work" to someone who already did the deed; for example, a wife would tell her husband this after she welcomes him home, or you would tell this your co-worker that the project you did with him was done successfully.
'(company name) に勤めています。' is natural phrase as 'I am working at （company name）'. You can use it as it is.
There are several kanji of ’つとめる’. Each kanji are a bit different.
The kanji what you write is a bit different from I write.
But you have known very well much kanji. awesome! :O
I have a general question. Don't japanese people get confused with so much stuff before the verb? In this case for example, I feel like you're just saying a lot of stuff that's piling up to finally explode when you say the verb. And when you get to it you don't remember half of the things that had relation with the verb, isn't it?