@karlenigmasorry I'm late here. I head up marketing for our company and we use NS for all of our marketing automation needs, including sending emails to multiple groups like you're trying to do. Lists and email groups are created through a saved search. We're B2B so our customers are companies, however, I always send emails to real people, not faceless businesses.
I always start with a Contact Search and pull in various Customer Fields via the Advanced Saved Search Criteria. When scrolling through the list of Contact fields in the search criteria, you'll see you can pull in other entity fields below the alphabetically listed contact fields. To do what you're trying to do, I include the Customer status and pull in the relevant Lead, Prospect and Customer statuses. I also only include certain contact fields, such as the custom Job Role/Buyer Persona field we created, to make sure we're only sending relevant emails to to the right contacts at those companies.
Then I create a dynamic email group and use that for sending individual campaigns so I don't have to create the same saved search again. I have tons of these that get used in marketing or informational emails as well as workflows to trigger emails and other actions.
Hope this helps! Marketing in NS can be a chore if you're not familiar with the process, but the level of granularity you get compared to other email providers is incredible. Let me know if you have any other questions and I'll do my best to answer them.