Hi Pavan,
If your requirement is just to assign user groups to a single user who exists in the Portal, you can do that through Identity Management by searching for the user and by modifying the assigned groups.
If the user does not exist in the Portal and you wish to import the user and assign the groups simultaneously, you can use the Import feature of UME. Using the import functionality, all the user information will get imported, including the roles and groups assigned. Please refer to the below format:
[User]
uid=userid
last_name=Lastname
first_name=Firstname
language=en
accessibility=0
role=pcd:portal_content/com.sap.portal.xxx;
group=Administrators;
Hope this helps!!!
Regards,
Anurag