You'll be glad to hear that the ArcGIS Online Subscription is included as part of our campus site license. We've been granting access to the subscription for no extra charge. If you are paying your annual ArcGIS charge, you're already paying for a portion of our campus site license.
However, we have a limited number of service credits (12,500 remaining at the moment). The administration tools for the subscription are pretty limited, so we ask that you use credits carefully. Here's a link to see how credits are used: http://www.esri.com/software/arcgis/arcgisonline/credits. In many cases there are desktop-based alternatives for the most "expensive" operations (e.g., tile caching, geocoding, analysis). I would be happy to suggest alternatives if you think something is looking expensive. We may also end up charging for the subscription at some point, but I don't anticipate that we'll do that in the coming year.
Once I've invited you to the subscription, you'll see an email notification letting you know how to connect. If you already have an Esri account, you can sign in with that. Otherwise you'll be prompted to create an account. If you already have a "public" ArcGIS Online account, you may want to keep it separate from the subscription.