If you want to build a grab and move application in grasshopper, then the first step is finding which object in your model is closest to a users hand in order to move that object. Begin by using the Brep Closest Point component to find the closest point on a brep to a given target point (we will swap this with the users hand later). You can then sort the distance to these points as well as the list of objects using the Sort List component. This component goes through a list of numbers and re-arranges the values in ascending order. By connecting our geometry to the 'A' input parameter, the Sort List component will also arrange our geometry in the same order - such that the geometry with the smallest distance to the point will be the first item in the output list. Finally, we use a list item to grab this first item.
To find the closest object to a tracked Click, replace the Point parameter with the location of the cursor at the time of a click. For a more advanced tutorial, see Moving Holograms in Grasshopper.