From the website:
If you have a large list of employees, you can import them to Sling using an existing XLS or CSV file.
Go to Dashboard > Employees and click the Import employees button, then click Import from file.
You can browse for the file or drag and drop it into the box with the dotted outline.
The next step allows you to label the columns in your file.
Note that a maximum of 7 columns can be defined when importing a file. If there are more, you will receive an error.
You can also choose whether to send the invitations at the same time or delay inviting your employees until later.
Finish by clicking the Import employees button and they will be uploaded to your employee roster within Sling.