They tell us, among other things, that Mr. Easton, this conman on his way to prison, was once a member of the social elite. This is the same Washington social elite of which Miss Fairchild is a member. She obviously knows Easton and regards him as someone you could describe as the life and soul of the party. That's what she means when she says that he's been missed. The "old crowd," i.e. the upper-class social group of which Easton was once a part, miss him terribly, presumably because he was always such good fun.
Of course, the reason why Easton's been absent for so long is because of his criminal activities. But Miss Fairchild doesn't know this yet; in fact, she'd never have thought in a million years that this distinguished young gentleman from the old crowd would ever get himself mixed up in any kind of trouble.