In your get verb, loop through the circles in client.screen until you find one that is not used, then set the obj on it.

You'll have to track which circles are in use. (You could check by comparing screen_locs of objects in contents, but that will be relatively slow as you get more objects and more circles.) I would give the circles a var to track what item is on them. When checking the circle, see if that var is set. If it's set, check to be sure that item is in the mob's contents (if it's not in contents, clear the var.)

This will also give you a way to make sure a circle is not already in use during a mousedrop.