When a new user is added, sometimes it appears that they cannot see any files.
This is not a bug but a feature :-). In order to protect the confidentiality of all of your files, new users are defaulted to zero access. You need to proactively give them access to Folders or Files. You can do this 2 primary ways.
1. Make them an Admin for the account. This is for super-users. Under Users: Settings, if you set their Role to Admin, they will have access to all of your files automatically (be careful!). You can do this by clicking on the box that shows their Role.
2. Set their Permissions. This is more common. Under Settings: Users, you'll see next to their email address a Permissions box. If the box says None, it means they won't see anything. You'll need to click on the box and choose what Folders (or Tags) they should have access to.