Just looked into that this morning actually. The INMG website says series registration doesn't close, but cost does go up shortly so the longer you wait, the more it will cost. I'd assume any matches shot would count, no matter when you sign up?
I was wanting to sign up and pay at the match Sunday but no one was there to set that up or knew how.
This is correct. Registration doesn't close, but price goes up April 1st I believe. I'm in San Diego right now, so I couldn't help much with that Sunday.