If leadership means making decisions, raising up new leaders means… delegating decisions.
If you want more people involved, you need more teams/groups for them to join. If you want more teams/groups, you need new team/group leaders. If you want to see and develop more leaders, you need to give them things to choose. You need to give them the opportunity to make some decisions. Slowly give them more and more authority and responsibility… until you have a leader.
This isn’t hard, it’s just risky. Say, “Hey, we need to work out a growth group social event, and I’d like you to decide what we do! The decisions up to you. I can give you some suggestions, and I can give you feedback on any ideas you have, but I want you too choose. Ok?”
Tip: make sure you give more than 2/3 suggestions and include things you WOULDN’T do.