how does the man in evening clothes know rainsford?

Zaroff knows of Rainsford because he's read a book about hunting that Rainsford authored.

In a cultivated voice marked by a slight accent that gave it added precision and deliberateness, he said, "It is a very great pleasure and honor to welcome Mr. Sanger Rainsford, the celebrated hunter, to my home."

Automatically Rainsford shook the man's hand.

"I've read your book about hunting snow leopards in Tibet, you see," explained the man. "I am General Zaroff."