Please log into the admin portal at admin.xogo.io (enter the same username/pass that you use for the XOGO Manager app).  You should see in the account > billing section that you have a credit for X number of dollars.  You can use that credit to "buy licenses" for your account and then you can add your players to XOGO Manager.  

Here's a little video from showing how that is done: https://www.youtube.com/watch?v=LsJhgZSoc4U