To add units into a group or remove them from it, use the command unit_group/update_units:

svc=unit_group/update_units&params={"itemId":<long>,
				    "units":[<long>]}

Note: The units parameter must contain IDs of both objects already added and that should be added. In the case of deletion, you must list the IDs of the objects that should remain in the group.

{
	"u":[<long>]	/* array of units IDs */	
}