In order to use the newly added companies in Sage 500, you will need to activate the modules to be used in each company.  So it's not really an activation of a company, but the activation of the modules to be used in that company.  Until you do that, you will not be able to do anything in most modules.  Plus, you may need to revisit user setups (SM | Maintain Users) to ensure that each user that needs to access the newly added companies can do so, as user setup can be restricted to particular companies.