Knowing Attendance in Advance

I was discussing with a Christian academic a "flop" that she had had with attendance at a special lecture. But a lot had been left to chance. Throughout my ministr(ies), I said, I have made sure that attendance is known fairly well in advance -- with the exception of Sunday services. But even there, attendance has been partly known in advance. OBSERVATION: Often, in my ministr(ies), the problem was over-attendance. There are various ways of knowing attendance in advance.