Note that Additional People are now only seen on legacy accounts. If you don't see it, you have a newer account.
Additional People & Linked Records are 2 different ways to group leads & clients together.
Additional People are usually people on the same policy or children that you wouldn't want to market to. Linked Records allows you to group families together, but still market to the individuals as Leads & Clients. A way to think of it is that "Additional People" is information on the primary policy holder whereas "Linked Records" is a way to organize/group related policy holders. This also can be grouped by companies, Referrals and more.
A reason to group as Additional People would be to only send emails, calls & marketing material to one person on the policy, but still keep track of their family.
A reason to group as Linked Records would be to market to multiple people within the family. For example, say Joe has Medicare Advantage through you. His wife Jane isn't yet 65, but she's a potential client. Add Jane as a lead and then link their records. That way, you'll be able to send emails for "Turning 65" and add her to your call queue for "Turning 65".
Click HERE to see how to create Linked Records.