by far the easiest method of adding xmodels (grass in-particular)

you must already have the chosen model loaded in radiant and have the model selected for this to work

with the model selected , press and hold ctrl + alt

then , left click anywhere in the 3D window where you want to place another

very fast and efficient

remember to have random rotation and scale toggle button selected on if you desire the above affect on model or entity , this also works for placing spawn points , very handy but you may not want to have random scale on for that

(tip credited to Paulo88 , Treyarch developement team , more tips and a more in-depth look at this tip from Paulo can be found on the RGN wiki)

http://wiki.modsrepository.com/index.php/Call_of_Duty_5:_Paulo88_Radiant_Tips

this was intended for W@W , ive tested for cod4 aswell , worked just fine , cant speak for other versions

Blade, my apologies, I thought that Ivan was the OP. I would have immediately answered your question first had I realized.

Here's the best way to work with grass models...Either you should make a prefab of a bunch of models that you can load and copy/paste or you'll need to select the model you want to copy, hold control and alt, and click in your map to copy the models to new locations. It's not extremely efficient unless you do it the prefab way, but it'll work either way. Thank you, Kyle Mulliger aka Deadly or Uzumakibro93.

PS My apologies once again!