Let's look at how to purchase Visual Studio Team Services (VSTS) resources for your customers.
If one of your customer needs VSTS resources such as additional VSTS users or the Test Manager extension, log in to the Partner Center, and choose that customer. Click Service Management, and then select Visual Studio Marketplace. Make sure that you're in the Visual Studio Team Services tab, then search for and click Team Services Users. Click Get, and move through the purchase process. Select the team services account that needs more users, and then choose the Azure subscription to be billed for the purchase. Next, select the number of users your customer needs. When you're done, review your order, and complete your purchase.
The next time your customer logs in to their VSTS account, they'll be able to start adding additional users right away. Purchase other VSTS extensions the same way you purchased additional users. For example, if another customer requests Test Manager extension, search for and click Test Manager in the Visual Studio Marketplace and go through a similar process.