It is possible to assign a device to a group during bulk registration. The name of the device group needs to be passed in a CSV column called "path".
The following CSV shows you how to assign one device to a group, and not another one:
ID,IDTYPE,CREDENTIALS,NAME,TYPE,SHELL,com_cumulocity_model_Agent,endpoint id,lwm2m server uri,securityMode,registrationLifetime,c8y_GenericUi,bindingMode,path
Result: someclient3 will be added to examplegroup.