Personally, I refer to shills as equalizers.

The purpose of a shill is to sit at a table and get people to play. The idea is that players won't start at a table but are more encouraged to join in a game that's already going.

They aren't "coolers" or someone who is inserted to change the game, but used to entise people to sit or play.

That is why shill bidding is refered to in auctions. Someone who either starts the bidding to make the item look more appealing or someone who raises the bids, to increase the values.

Originally the shill was used to dupe people into getting involved in a swindle.

Modern example some street vendor selling junk. The first buyer or two, step up and get the buying started.