Hi Jatinder,
Below is a link to a similar error that will have the same cause solution for you.
If you go to: System Definition" -> "Tables" and search for "Name" = "sys_user_group".
If you then open the table and look at the user columns like Primary_Lead or Group Leads, you will see the fields are reference to the 'user' table so it is expecting the sys_id of the user from the 'user' table.
If you are trying to add the user as a member to the group that would be table sys_user_gmember if you look up this table and look at the Group and User fields, you will see they are both references and you would also need to use the sys_id there too for import.
Similar issue to review: 'Import Error - Multiple records for Question exists in table'
https://www.servicenow.com/community/now-platform-forum/import-error-multiple-records-for-question-e...
I hope this helped. Please indicate if it did.
