For showcases do you simply get invited? Or must you sign up and register then they pick and choose who will come and who will not? Thanks.

Most showcases will simply and happily take your money

There are a few very selective showcases where they invite a very high level player such as the Under Armour All-America Game, Team One Nationals, AFLAC, Perfect Game Nationals, Area Code, etc.

The individual showcase will let you know as they register you for their event, if they do not specify it is an invitation only, most likely they will take all comers.

Some may limit due to the facilities and the amount they can handle on a given day or two and still provide a quality showcase.