^^^^ This would be your best bet OR you could go in every single thread in the Midwest: Discussion & Matchmaking and copy down EVERY SINGLE NAME you see.
Seriously though post up in the tourney section. close thread…
If you want to personally invite people, gotta go through each of the threads. Otherwise just do what everyone is saying and just make a thread in the tournament section.
Anyway, spamming (and thats what it is) out private messages to everyone to nag them (thats what it will feel like) to come to your tournament isn’t going to be encouraged. I’m sure you’ve noted that from everyone’s responses already.
SRK has a tournaments and events subforum, in which you can post about your event. After that, most tournament directors just leave a link back to the tournament posting in each of the midwest discussion threads here.