Moses encountered this problem with the Israelites in Numbers 11. It was only when his focus was returned to its proper place--on God--that he was able to rally the Israelites around God's work.
In the same way, Jesus' disciples prevented ministry done in Jesus' own name because they didn't "know" the person involved! Jesus corrected them by reminding them that if someone is not against him, they are for him. This is a lesson the church needs to learn today.
The solution to the complaining problem is commitment--instead of holding back God's work by complaining and criticizing, pitch in and find a way to contribute. Together, we can build the kingdom Jesus proclaimed.
Sermon: Complaints vs. Commitment