We start putting creep out when the calves are about a month old. I don't know what you would call a "large group", but we usually start 10-15 calves on creep at the same time. The calves are usually checking out anything they can get into by the time they're a month old, and they're investigating just what the heck that creep feeder might be. We put out some very sweet feed and a little hay in there at first, and one or two calves will get it figured out maybe that first day, and a week later all the calves are scrambling to get there first when we bring the buckets out.