The easiest way to combine two or more lists is to build a segment that grabs contacts from the desired lists.
Alternatively, you can export one of the desired lists and then re-add it directly into your other list. Once you do this, you can delete the first list completely, since all of the contacts will now be in your new combined list. Just remember to switch the list ID of any sign up forms connected to the old list.
Using a Segment to Combine Two or More Lists
The most common use-case for this is when you have one primary newsletter list, but then create lead magnets on your site to encourage more signups by offering incentives. A single person can only subscribe to a given list once. This means if someone already on your newsletter list is interested in the incentive you're offering through a separate lead magnet, he/she will not be able to opt-in again if this lead magnet is also linked to the same newsletter list.
When you create different lists for each lead magnet, however, you will likely want to combine all of these different subscriber lists so that you can send your regular newsletter communications to your entire subscriber-base. This is when we recommend using a master segment. The following image shows an example of this:
Be sure to use "OR" connectors to grab contacts from any of the desired lists.