Returns a list of all users who have access to a given block. Depending on where the block is located, there will be differences in access levels.

• A block in a Teacher Key can be granted admin or read access. By granting admin access, you make that user a Teacher. If you granted read access, the user will become a Student.
• A block in a Project Key can be granted admin, write, or read access. If you want the collaborator to be able to share the block in turn, grant them admin access. Else, you can grant them write, or read.