Hello @froda,
First, welcome to Klaviyo! We’re thrilled to have you on board. And thank you for sharing this question with us in the Community.
All of the methods for adding a user to an existing list are outlined in our article here. This includes uploading users to a list via a .csv file (which will not create duplicates if they already exist in Klaviyo) and also adding them via the “quick add” button. The former method is better if you’re looking to add a lot of contacts to the list and the latter is better if you’re looking to add just a handful. Both methods can be applied to existing contacts in your Klaviyo account. I also recommend checking out our article on adding users to lists here.
An additional way to capture individuals who already exist in Klaviyo is to create a segment. Segments are groupings of contacts defined by a set of conditions. For example, if you wanted to capture individuals who placed an order in the last 7 days, you could achieve that with segmentation by using What somebody has done > placed order > in the last > 7 days. Similarly, you can use segment criteria to group lists together. For example, If somebody is in a list > is in list A OR If somebody is in a list > is in list B. That would combine the users in List A & B into a segment (and filter out any duplicate profiles). Segmentation is the most powerful and popular way to group already existing users based on some pre-existing behavior.
I also recommend reviewing our AND/OR Guide to better understand what these mean because they are important concepts in segmentation. In short, with an “OR” criteria someone only has to meet one of the joined conditions in order to be added. For example, if someone is in list A OR in list B, if the user is a part of either list, they will be added to the segment. As opposed to “AND” which requires that the user meet all conditions separated by “AND” to join the segment. For example, if someone placed an order AND is in list A, they would have had to place an order and also be a part of list A in order to qualify for the segment.
I hope this is helpful!