A colleague was issued a 10-year B1 visa to travel to the US from India while at another company. Can he still use this same visa (within the 10 yrs of issue) for business travel with his current company? Or is a new visa with invitation and business letters from his current company required?
Simple rule, if the employer's name was annotated in the visa page, then you should apply for another visa.
If the employer's name was not annotated there, then you can use the same visa, just make sure to bring what proves your business.
In general, the visa is issued to the person himself to conduct certain businesses (short training, meetings, negotiating contracts, etc). So even if you go to the US while with your current employer, you should present some documents for whatever business you are conducting there. In addition to that, nothing is mentioned in the official B1 visa page about changing the employer.
P.S. I had first hand experience since I am a B1/B2 visa holder.
Here is a copy (after modification to hide some important info) of my old annotated visa, my company's name and my job was clearly mentioned.
This is the new visa, it has no annotation whatsoever, and I used it to enter US to attend a seminar, I had to show the invitation letter at the airport anyway: