Make sure and review all Update Notes.
When upgrading from version 8.1 or earlier to version 9.0 or later, you may have Users that are of User Type "Rep". This User Type is no longer supported. In order to access these users in System Management | Users, you will first need to run the following query in your SQL database:
Update BDUsers
Set UserType = 0
Where UserType = 1
This will set all Users of type "Rep" to type "Contact". If you want any of these Users to have access to any of the Enterprise modules, you will then need to go into System Management | Users and change their User Type from "Contact" to "User". Next, go to the Roles tab and assign the particular roles that you want them to have. Caution: User types of "User" count against your number of licensed users.