Welcome to the community @Percival 

To create a segment of the users who didn’t complete the double opt-in process and so have not been added to your list:

• Add a custom property to your form that gets added when they hit submit. For example, that property could be subscribe_submit and set the value to true. That will tag any user who submits the form.
• Create a segment of users who have submitted your form but have not been added to your list (because they didn’t confirm), follow my example segment below. The list to include is the one your form submits to.

Does that answer your question?

Andy

Hi Andy,

Thanks for your help with this.

After reading your response I realized I have a custom property:Value already set “Source:challenge reg”

So I created a segment as per your screenshot with this existing property and I got 48 members.

Is it possible to see when these 48 members signed up?

Thanks.