Hello,
You can create a checkbox vendor field and make it as required.
In the checkbox label, you can add a link to a Joomla article or insert special tag to use a popup system provide by your template.
By showing that field during the registration, you will have your "terms of service" acceptation.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.