Select the List you want to add the subscribers to and select Manage.

Select Subscribers

Select Import

Click orange Import button

Make sure your file is a CSV. If you need a sample file, select Download a "sample import file" which is located under the Requirements box.

Choose your file and select Open.

Click Next

Select Subscribe New Numbers

Select the data you'd like to include by checking/unchecking each box.

Map your data by selecting your property in the dropdown for each field you are mapping.

Check the Confirm box.

Click Finish

File will upload. Large files may take some time.

Click View Report to see how many numbers were imported.