I've never made it back to the pack myself after a flat. But I once dropped back with another rider (or 2?) from my team to help our leader chase back on. With three of us (maybe it was 4 of us?) we were able to catch back on without drafting. That effort just about ended my race, as I really didn't have legs after that to do much but sit in. And our leader didn't end up doing all that well partly because of the big effort to get back on.