Hi,
A vendor is an entity and can have several users to manage it.
If you give the ACL "vendor/edit/users" you authorize your vendors to manage their users.
To add a user, the "vendor manager" must know the email address.
If he enter an email address which does not exist (when there is no Joomla account associated), the addition will be refused.
And by moving your mouse on the blue " i " icon ; you will have a nice tooltip explain that.
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.