Some insurers give concisions and discounts for named drivers. They usually do when its either a spouse, or an older driver on a policy with a young driver.
With additional drivers you need to ensure you are asking all the relevant questions. They might end up being a determinate to the customer and there ability to obtain insurance based on prior history.
Different driving restrictions -
IO Insured only - Covers policy holder solely
IS Insured and spouse - Covers policy holder and married partner
ND Insured and named - covers policy holder and up to 4 additional named drivers.
25/30 Any driver over 25/30 - covers any driver over the stated ages as long as they have a full UK driving licence and answer no to all of the rating questions such as Claims and convictions.