They weren't late, though? If they wanted people to show up 5-10 minutes early, make the training at 5:50 or 5:55. Don't say 6 and really mean earlier.
Asking participants to arrive early would, at least to me, strike as a caveat to those who might otherwise arrive late -- but not at the "time in question".
On time is on time, which is AT the stated time. Not before, not after.