Follow these 8 steps to create many new members at once:
Go to the member page.
Click on the "+" next to "Members" at the top of the page.
Click on "Create many members":
Choose either "Semicolon" or "Comma". Start with semicolon if you are unsure. If it looks strange after the upload, then you need to choose the other option.
Click on "Upload CSV".
Then name the columns:
Click "Next". Here, it will show if there are problems with any of the "members". In this following case, it is a matter of having blank rows and rows with headers, which are not members. I simply ignore those. But check if there are any errors.
Then click "Next", and the members are created:
Read about what a CSV file is here.