How to remember many names?

During work, I meet many persons; usually a group ranging from 2 to 60 or 70. I find it hard to remember their names and recall when I meet someone afterwards individually. I end up asking their name again.

I vaguely remember I had a friend who recalled 40 names in the first day of the class just after introduction. I don’t know how he did it.

What are suggestions? Thanks in advance!