CloudCompare forum

You should be able to compute the cloud to cloud distance and cars should have higher values. Than you can use filter by value to segment the cloud.

There is also a CSF plugin that can be used to separate on-ground and off-ground points, if the cloud to cloud distances don't d what you want.

For larger clouds, it may be worth exporting from CloudCompare and importing into the free version of Autodesk Recap. The .rcp format from Recap pulls into AutoCAD somewhat easier.