Before sharing a collection, you need to create one from the collection browser.
Clicking the "Collections" button will bring you to the browser and in there you can create a new one in the top left corner.
After clicking Create Collection you will be taken a another screen where you define the rules of your collection.
Set a name for the collection.
Set the visibility of the collection.
Add members (users) to the collection.
If you want to collaborate with guest users you have the option to add them as well.
Now that your collection is created you will be taken to the workspace of that collection.
If you click the name of your collection in the top left corner a drop-down menu will appear.
Under Collection Options you can change background, get a sharable link for your collection, rename it, archive it or delete it.
If you click Manage Members you can change visibility of the collection, invite other users to your collection and manage permissions.
By clicking Invite external guests you can invite people from outside of your organization to the collection.
Note: We have made changes to the sharing functionality (December 2016) so it is no longer possible to share individual widgets (boards & backlogs) within a collection to specific users.
If you have widgets shared to specific users within a collection there will be a orange triangle on that widget.
If you click the triangle you will see which users that specific widget is visible to.
The visibility of the widget stays the same, however it is not possible to create such a setup anymore.