When creating the list of users in Excel, they should be presented as shown below:

Once the CSV document is completed to your liking with all necessary users, navigate to the Management Tab where you select the “Add Members to Distribution Group” action

You will see a “Load from CSV” option as shown above which will allow you to upload the Excel document previously created.

Once the file is shown, a list will populate allowing you to verify the Users you would like to add to the group.

You will then be able to select the Distribution group the users should be assign to and proceed with the action.