If you want to import multiple contacts all at once, the easiest way is to add each contact on a different row in your sheet. This is true even if you have multiple contacts that you'd like to import to the same box.
Be sure to use the same Name data in your Google Sheet that will match to the box's Name in your pipeline. The import tool will see that there's multiple lines with the same name and know to combine this data together when used in tandem with the Update existing box with new data option in step 6 of the import process:
If the Sheet has multiple email addresses on the same row in the same column, the Import tool will only import the first email address listed in the column.