Almost everything ever written about friendship is written for two people. Send the text. Make the first move. Ask again. Be more vulnerable. It is decent advice, and it quietly assumes that the unit of friendship is a pair. For a lot of adult social life, it is not. The unit is a group — five people in a thread, the ones you used to see every month, the table at somebody's birthday. And groups do not fail the way pairs fail. Nobody falls out. Nobody stops caring. What happens is duller and much more common: the plan does not get made. Six months pass. Then someone says the group drifted apart, which sounds like a story about feelings and is almost always a story about logistics. Here are four ways a group quietly stops working. None of them is...