How can I add bots to an Alien Swarm online game?

To add a bot to your game: cl_selectm (marinenum) (slot)

To remove a bot from your game: cl_dselectm (marinenum)

(marinenum) should be between 0-7 and corresponds to the character to be added.

(slot) should be between 0-3 and relates to the player slot the bot will fill. -1 can also be used; this will fill the next available slot.

These commands must be used while in the lobby; using them anywhere else will result in invalid command errors.

Source/More information