They need the free OneSpace app or the free web client, and never a paid account. Receiving files is always free.
When you send to someone who does not use OneSpace yet, they are invited to open the transfer in the web client (nothing to install) or to install the free app. Your files stay end-to-end encrypted either way: no readable copy ever sits on any server.
Still stuck? Email support@onespacesecurity.com from the email address on your account. A real person reads it.