Encourage them to do something new, something slightly uncomfortable (yes, I mean team-building, but not in a naff, unimaginative way). See if you can get them to:
🕹️ Go for a walk together (and talk to someone they don't normally talk to).
🕹️ Go to watch a stand-up comedy show.
🕹️ Design a board game.
🕹️ Anything that could incorporate silliness.
Baby steps. They will probably resist, but if they don't take any risks then they will stagnate.
Staying in their comfortable safe place where nothing changes is not a good strategy. Confidence is a muscle that needs to be used and strengthened. Help them to do this and you might see some wonderful improvements in mental health, productivity and communication skills.
And profits, of course (use that to sell this idea to your boss).
[Image by Dan Carlson on unsplash.com]