You can send a personal event link to any guest. Send it to people you've already entered as guests to your event.
Personal event links exist in order to make it as easy as possible for guests to respond to invitations, especially when they have not used Early Bird previously. Each link is associated with a specific guest of a specific event. When a personal event link is clicked, Early Bird registers the email address as an Early Bird user, setting the user's name and email address to be the name and email address of the guest - and can then email a login link to the user. If the user is already registered, then Early Bird just emails the login link to the user. After logging in, the user is taken to the event home page. If the user was already logged in when the personal event link was clicked, the user is taken directly to the event home page, without needing to receive a login link.
Personal event links should not be shared with other users, as they can only register and log in the guest that "owns" the link. But doing so doesn't create a problem. The user will just be directed to the event home page, as if they had used an event link instead of a personal event link.
You can send an event link to anyone. If your event is open for RSVPs, anyone with the link can become a guest. The workflow is not as simple as with personal event links, but it is pretty simple. And if they log in with Google, they can avoid dealing with login emails.
An event link sends the user to the event home page. If the user is not logged in, they are taken to the login page, where they can log in or register.