It seems that you can't share a "Document Management" folder until a "pending" user logs in for their first time.
Current Workflow: Drawings need to be issued to a bunch of people immediately. One team member is not currently on BIM360. You add that external team member to BIM360. Right now they are "pending". Go to your project. Add the team member to the project. They are still "pending" and their name is "New User". Go to Document Management. Try to share a folder with that "pending" team member. You can't do it.
So you end up having to wait a day or two until they finally log into BIM360 and set up their account. Then they go to the project and can't see anything and start saying how bad BIM360 is. A few hours later when you finally can get to your desktop, you log in and share the folder with them so they can see the drawings.
That workflow needs to be improved. The "pending" user should be able to be assigned folders and then have them available immediately the first time they access the project.