So, to add a bot to your game:
cl_selectm (marinenum) (slot)
(marinenum) being numbers 0-7 for Sarge, Wildcat, etc. through Vegas, respectively.
(slot) being 0-3, for players 1-4, respectively. Slot number -1 can also be used to add to the first available slot.

To remove a bot from your game:
cl_dselectm (marinenum)
(marinenum) again being numbers 0-7 for Sarge, Wildcat, etc. through Vegas, respectively.

These commands must be used directly from the lobby menu. Using them anywhere else will result in invalid command errors from the console.

For those suspicious of the ability to add bots to a game, let me clarify some points:
-- You do not gain any EXP from bot kills and bots simply share your level.
-- Bots can, unfortunately, be added by public game joiners. However, new players can still join, and once the lobby fills up, all bots will be automatically kicked and none will be able to be added.
-- When a player leaves (or is kicked from) a lobby, his/her bots will leave as well.

There are, of course, some issues with MP bots as well. Anyone giving a "hold" or "go" order will order all bots, regardless of rather he/she "owns" them or not - however, equipment orders and marine switching work as intended. Also, bots sometimes do not leave correctly when new players join until the lobby fills up to 4 players, as mentioned above - this is being looked into, but in the meantime, use the cl_dselectm command to remove bots if you don't have a full lobby joining.

Source: http://forums.blackcatgames.com/showthread.php?t=12996