Everyone within the group that has the app will receive a push notification when you send out an invitation (As long as they are invited).
The exception is if someone has turned off push notifications for Spond in their phone settings, or turned off 'Invitations for events' within 'Push notifications' under the tab Me in the Spond app.
Those that are only registered with SMS will receive an SMS, while those that only are registered with e-mail will receive an email with the event invite. Those who have not yet accepted the group invite will receive a reminder and need to accept the group invite before they can respond to the event.