I assume you have managed to get yourself an area full of grass clumps.
Now then: go to the object tab and select "pop grass clump".
Tick: "use density shader" and left click the button with the three ...
Then: create new shader: surface shader: image shader.
The image shader should act as a mask to get the grass-clumps in the areas you want:
white area will produce grass-clumps, black area: no grass-clumps.
So for practice you can make an image of a simple circle-shaped black area in the center of a white color, use Photoshop or a similar program.
When you've imported the mask-file: tick the three ... again and choose: "go to image map shader".
There you choose as your projection type: Plan Y.
Tick: position center.
Size: same size as in the "pop grass clump" where we started, 1000 x 1000 for example.
Hope this will help.
Greetings.