What are they and why are they helpful?
Praying, of course, is not just an individual pursuit. It can be about praying for others and the concerns of community in which we live - locally, nationally and internationally. Praying for others, or 'interceding for others', is like standing in the gap between God and his people, offering to God the concerns of the people, and vice versa, offering to the people the concerns of God.
There have been many books written offering prayers for God's world, prayers for others, and prayers for ourselves, which you can use to lead your group in prayer. You may need to spend a few hours in a bookshop/online looking at different styles/types of prayers that will be helpful to you in your own context. Remember that people can enter into prayer not just through the spoken word but also with many multi-sensory approaches.