If you'd like to send a resource to a specific group within the class, you must use the Students filter on the top bar, explained here : How to: Filter students in the grade book
Here a short video
You can add resources from this very same screen with a tap on the small paper clip icon.
Send an e-mail with a single resourceYou can also send resources by e-mail from any other view in iDoceo
Tap and hold on the resource > Export > Mail students icon