If you want to reactivate an inactive user you need to be a System Administrator and follow these steps:
 

System Administration --> Organization --> Users --> check the "show inactive" flag --> click search --> Click the user you want to reactivate --> Click Reactivate.
NB! Check that the user has atleast these 2 licences:
Platform User (fw)
Business Analyze for SuperOffice licence.

See screenshots of how to do it: