Hi,
There is no direct method to select or even reference PointCloud geometry directly into Grasshopper.
You can check out Cockroach Plugin which has a component to select a pointcloud like how you would reference any other geometry.
If you would like to dig a bit deeper, here is a pythonic version of importing point cloud from Rhino into GH.

""" Imports all pointclouds in the current Rhino Document.
Input:
select <bool> : ItemAccess
Output:
clouds <list[Rhino.Geometry.PointCloud]>
"""
__author__ = 'Kaushik LS'
import Rhino
clouds = []
if select:
for obj in Rhino.RhinoDoc.ActiveDoc.Objects:
if isinstance(obj, Rhino.DocObjects.PointCloudObject):
clouds.append(obj.Geometry)
If you’re looking to select a specific point cloud, use this then
""" Implementation of Get PointCloud(s). Imports selected PointCloud.
Input:
select <bool> : ItemAccess
Output:
clouds <list[Rhino.Geometry.PointCloud]>
"""
__author__ = 'Kaushik LS'
import Rhino
clouds = []
if select:
result, ref_obj = Rhino.Input.RhinoGet.GetMultipleObjects('Select PointCloud(s)', False, Rhino.DocObjects.ObjectType.PointSet)
if isinstance(result, Rhino.Commands.Result.Success):
clouds = [obj.Geometry() for obj in ref_obj]
Edit:
Check attached file to use it in Grasshopper Player.
FIle: select_pointclouds.gh (3.9 KB)

–
HTH
~ ~ ~ ~ ~ ~ ~ ~
Kaushik LS
Chennai, IN