If you would like to just say “work for (company name)”, you can say:
- 私は(company name) で働いています。
- 私は(company name) に勤めています。
Both are natural. I think 2 is little bit more formal but 1 is also not so casual word.
On the other hand, “(company name)に仕える” sounds incorrect if you simply work for a regular company/public organization. Because “仕える” means “work for noble people/god” so you can only use this word in limited situations like those below:
「私は神に仕えています」(she must be a sister and works at a church (in the case of Christian))
「私は王女に仕えています」(you may can imagine how rarely people work for queens)
In the old age in Japan, about 400 years ago, Samurai worked for their domain/shogunate and that was called “藩/幕府に仕える”. It has shown that were absolute being at that time. Unfortunately we contemporary persons cannot normally use “仕える”.
If you said “(company name)に仕えています”, I would laugh and say “Hey, are you a knight or something of the company?”.
Hope this helps you.