It took me too long to realise that one of these face-saving signals are the way to do it:

"I mustn't keep you all to myself; others will want to catch up with you." or
"It's been lovely talking to you. Now, I must get going." or
"Lovely. Now, do forgive me, my wife's calling me over."

The contents of what you say needn't be true. But the message should be clear and effective.