Once you have a collection created, you can assign it to a Group so that its member can access its content. To do this, navigate to a Group and click on the Unity Collections tab. Use the dropdown menu to choose which Collections they should be able to edit and click on the Assign button.

To allow Discussions in the Group's Unity portal, click on the Unity Social tab and enable Allow Group Social in Unity. After it's enabled, you'll have additional options to allow members to post discussion threads as well as see a list of all members.