invite bots that can be replaced with humans

At World of Card Games, you can play with bots (against the computer), or with humans, or with a combination of these.

When you first arrive at an empty table, you will see a message to the right of your avatar. It says "Click here to start game with robots. Humans may replace them later." If you click this message, bots will fill all the open seats and the game will start. If a person wants to join the game, they are allowed to do so.
Click here to start game with robots. Humans may replace them later.
There is another way to invite bots to the table. If a seat is empty, you will see a small gray bot next to it. Hover your mouse over that bot, and you will see a tooltip that says "Invite Robot":
Empty Seat - Invite Robot
If you click there, a bot will join the table, but it is not replaceable. A human cannot take that seat, unless and until you or someone else clicks the message which invites replaceable bots to the table.

So if you want humans to be able to join the table later on, make sure to click the message about starting the game with bots.

There's an additional page about playing against humans and/or the computer using special options - take a look for more information there.