How do I deal with cliques forming on my team?

Cliques have set in on your team — in-groups and out-groups, people who are in on things and people who are left outside. The social lines have hardened into something that quietly decides who gets heard and who gets overlooked. It is corrosive precisely because it is so rarely said out loud. Everyone feels it; no one names it. As the leader, you cannot force people to like each other, and you should not try. But you are responsible for the conditions the team works under, and right now those conditions reward being inside the right circle over doing good work. Left alone, this hardens further, the excluded disengage or leave, and the team's real thinking narrows to whoever is in favor. Start by breaking the patterns that let cliques run the room. Change who works with whom on projects, make sure quieter and outside voices are deliberately drawn in, and watch closely whose ideas you yourself reflexively favor. You are not policing friendships. You are making sure being heard depends on contribution, not on which group someone belongs to.