This serves well for both community in the youth group as well as building them spiritually! They will love it!!
Simply order enough pizza for everybody coming (have them chip in if need be). For the first hour at least, have hang out time and pizza eating! This also includes playing some games. Then for the second half, have one full hour where the group has prayer. To make it effective time, schedule out different kinds of prayer (praying alone, for others, with each other, ect.) so that they can learn different ways to pray, and so time can seem to move along swiftly. Also, keep them informed the whole way, so they know what is going on. Your teenagers will be amazed at how quick, meaningful and fun just one hour of prayer can go!