Another grave mistake is having the wrong people.
When I first started exploring the idea of planting a church, the idea of NOT allowing anyone to come along seemed absolutely unChristian. How could a church turn anyone away?
The idea is not to turn people away, but to sell your vision and only take on leaders who assume your vision. Unfortunately, discovering whether or not they share your vision is a long process. Following my resignation from the church plant, I discovered that some had no intention of ever supporting my vision. After SIX YEARS!
Here are some ways I think you can tell if they share your vision:
1. They are already doing the things you want to do.
2. They begin to actively use your language and share the vision on their own.
3. They bring their friends.
4. They seem to be "internally" motivated in the same direction. This does not mean that they no long need your support or encouragement or reminding about the vision.
5. They are not denominational people or long-time Christians. This is not to say that people in these two catagories do not make good church plant participants. But, you should be very careful. Often denominational people want you to be like every other church in the denomination and long-time Christians don't have many unchurched friends.
Share your vision widely and often. Be sure to interview the people who join the team of core players. Choose wisely (or you too may shrivel up like that guy in the Raiders of the Lost Ark movie). Choose people who understand (at least somewhat) the difference between being a church and being a church plant (hint: there is much more required of the average participant in a church plant. AND, they will not have all the ministries they want either).
Don't be afraid to Dis-Invite people to the core team. Just like God prunes us to help us grow, so leadership has to be carefully pruned.