I have added a new user to the Azure active directory which is associated with Azure subscription. The user is added with the Global Administrator role.
However, when the user tries to login to the portal, no subscription is showing up. Tried logging off and logging in multiple times, but the subscription is not showing up.
Having rights to Azure AD doesn't necessarily give you rights to an Azure Subscription; they are separate. You can either Elevate access for Global Admins to manage your Azure subscriptions, or you can go the the Azure Subscription IAM settings and add the new user to a role.