We always recommend that your contacts sign up to your contacts list´s trough the in-app forms and pages, but sometimes you may have to add one manually, and this is how you do it.
Its very important that the person your adding have given consent, as this is required by most spam laws.
In order to add a contact manually, you need to navigate to the list you want to add to.
Once your on the list, you can click the "Add contact" icon in the top right corner.
A new form will open, which includes all the fields on the specific list your adding to. Fill out the form and click "Confirm".