Not able to add User by system administrator, Add button not showing on Ribbon to add user

Hi,

I do not see the option to add a new user. In record menu only shows edit, enable and Disable button but not showing Add button.
also I am trying to import users record through system administrator, but It's giving error "You do not have sufficient priviledges to import user records".

Can you clarify what is the problem?

Thanks,
S Deshmukh