User accounts can be given permissions to workspaces but not directories. This how you convert a directory into a workspace.
When logged in to the site as a Site Administrator do the following on the Files section:
1. Select the directory
2. Click the Access tab
3. Click Add Access.
4. In the drop down select the user to grant access to.
5. Adjust the permissions if needed.
6. Click Save
This will convert the directory to a workspace and give access to the user account selected.