The Extradium 2013 User Registrations list uses the SharePoint 2010 user interface (because of the icons used for the Invitation Sent, Registered and Approved custom fields), which disables the Quick Edit/Datasheet view.
If you need to bulk import invitations, please follow the steps below:
1. In the User Registrations list, select Create View in the SharePoint List Ribbon
2. Click on Standard View and enter any name (such as "Import View") for your new view.
3. In the Columns section, uncheck (at least) the "Invitation Sent", "Registered" and "Approved" fields and press OK.
4. Your new view appears. Select the wheel/settings icon in the top-right corner of your page and select Edit Page
5. In the web part drop down menu, select Edit Web Part
6. In the web part tool pane, expand the Miscellaneous category and in the JS Link field, enter clienttemplates.js
7. Press the OK button and select Stop Editing in the Page Ribbon.
8. Select edit this list right above the list. The list should switch to the Datasheet/Quick Edit view where you will be able to copy/paste the list of to-be-invited users.