The new grass extension in Maxwell Render 2.7 allows you to quickly add grass to selected geometry and has many parameters to control the look of the grass. Its implemented in SI as an object property, which allows you to have as many separate grass "components" as you want, attached to one object.
To learn more about the Grass extension, please see the Grass page in the Maxwell Render documentation, and the Grass Examples in the Knowledgebase for some example settings and materials.
Applying grass to an object
Select the object(s) and go to Property>Maxwell Grass. A new custom property set will be added to the object, along with a hidden point cloud parented to the object (used for displaying the blades in the viewport using SI Strands). Changes in the grass parameters such as density, length, nr of points per blade, and any maps you add to a parameter, such as a density map, will be reflected in the viewport display of the strands.
Maxwell Grass previewed in FIRE
Applying materials to the grass
A material in SI must be specified by name, and it must be attached to an object or the material won't be exported to Maxwell. For these reasons, you need to apply the grass material to an object in the scene (which should probably be out of view). Then open the MaxwellGrassAttrs parameter set, click on "Pick Material" and select the object with the grass material applied to it.
Each MaxwellGrassAttrs parameter set can use its own material and settings.
Mapping grass parameters