you have to watch out for the bacteria that can grow in sun-tea. it shouldn't matter if you use loose tea or bagged, but use real tea, not tizanes, the caffeine apparently keeps the bacteria at bay for a few hours.
also, never make more sun-tea than you will consume that day. it's also a good idea not to let it sit more than 4 hours in the sun.
overall, for a larger group of people i'd recommend either making fridge-tea with cold-brew bags, or just making regular tea and diluting and icing it down.