This page should answer both your questions: View Server Licenses
1. Licenses are managed through the Tableau Server interface, on the Licenses page, rather than your customer portal at tableau.com.
2. Licenses do not correspond to the user roles in Tableau Server. If a user is licensed as Interactor, site admin, publisher, etc, it just uses 1 Interactor license.
Hope that helps!
Shashank - I would recommend to check with your server admin to deal with this .If it's a desktop license i would recommend to add the user's name and email address .If server ,the admin will add all licences under his name and grant to user's when ever required.
I believe you made few user's unlicensed to grant others the license , that's the only standard practice for now.
For the Customer Portal, I would recommend assigning the Server Interactor key to the email address of the person who is in charge of managing the Tableau Server environment (Server Administrator). Each individual license key can only be assigned to one user in the Customer Portal, whereas the individual interactor seats are managed in the Tableau Server UI as Carisa mentioned above.