You need to help your teammates get better, sometimes in what they want to get better at, and sometimes in things you believe they need to learn for the sake of their team. This can be accomplished by pairing them with someone more experienced in an area they want to get better at, getting them access to websites like PluralSight, etc. Sometimes by mentoring them yourself. You have to give them goals, ask them to experiment with new things, help them write articles, give talks, and so on.