Groups are an entirely different situation than soloing. You have to change how you play to match your groups composition. If you have extra champs you may want to bunch the bad guys up and interrupt/AoE like crazy as much as possible. If you have burgs/LMs you want to CC and single target DPS when you can (hunter - stop spamming rain of arrows, please). If you dont know how the other classes play, you are ill prepared to figuring out how to adapt to your new groups makeup.