In the top right corner of the screen, click Administration, Expense, then User Experience Settings. From there you have the two options with big convenient buttons that you found, and between them are two other options that are little inconspicuous links. From there you can assign roles like you would any other.
After you've searched for and selected the user or group you want to change, scroll down to Expense UI Preview, click Add, then click Save. The things you have to do are not laid out well IMO, so here they are.
p.s. I find it hilarious that Drag and Drop is available when posting in this community and not in the new UI.
