The steps in this article require using an extid on user profiles. (You could, however, also do something similar with a unique, identifying user var.)
If you have a situation where you cannot, or do not want to, get the user extid by making a GET call to identify the user by the horizon cookie dropped by Sailthru, you do have the option to pass it as a UTM parameter in links.
To accomplish this, add the following to the UTM field of the setup section in your template/campaign:
Adding the above to the UTM field of your template/campaign will add this extid parameter to all links in the template/campaign.
An example link looks as follows: