To add new users/agents to WPL you need to first add that user to WordPress. Simply go to Users->Add new and add the user. Then go to WPL->User manager and click on the "Tick" icon for adding the user to WPL as an agent.
Note: In newer versions of WPL, users that have not been added yet, won't show by default. You should search them to see them. Please find below figure.
Note: WPL Membership addon has a feature for automatically adding all new users to WPL users. Please check this KB article.