The easiest way to invite non-existing users to create free sciNote account is by clicking on your profile and selecting Invite people to sciNote from a drop-down menu.
A new window appears, where you type in one or multiple emails of people you want to invite. Press Enter on your keyboard to add an email. Skip checking the box Invite users to my team and simply finish by clicking the Invite user/s button.
An email with instructions on creating a free sciNote account will be sent to them.