Here are some of the key points which has to be followed via the Import Tool
1) For On Premise, User Name must have Domain name. Format – domain\user or firstname.lastname@example.org format. For Online, Primary Email is mandatory
2) First Name and Last Name are mandatory.
3) All the users will be disabled in CRM.
4) We cannot set Business Unit during the import. By default, all users will be assigned to the Root business unit. After the import, we can update the business unit using Change Business Unit.
5) Every user will be assigned to Sales Person role by default.
Hope this helps.