To add contacts/subscribers to your newsletter, go to Contacts →Actions→ Import Contacts.
You have a few options of what you can do at this step.
1. You can upload a CSV file of your contacts.
2. You can integrate with another platform. We have quite a few native integrations with many CRMs. If you would like to learn more about our integrations, visit this page! We even have a rich integration with HubSpot which is available for Pro and Enterprise clients.
If you have one or two contacts that you want to add to your newsletter's subscriber list, you can add contacts individually.