Where can I find this? I look in Hammer in objects and I can't find this entities, how am I going to make a map if people can't join it? :S

the join marine, join alien, etc images don't appear in the ready room :s

that is a brush entity too...

a detailed how too make a join team sign guide:

make a brush rectangle of the right size...
the texture should be null on all sides
use the texture application tool from the menu
choose the surface and the appropriate 'join' texture
choose orient to face (or it might appear backwards)
hit apply, and fit to size
now right click the brush object in any veiw (except camera =( )
choose tie to entity
from the drop down box choose func_illusionary
now from render property choose additive

Hope that helps.. btw please read the turorials that are at the top of the forum as they cover very simple topics such as this.