Any named driver can use your vehicle occasionally. That means they shouldn’t be the permanent driver, nor should they be regularly using the vehicle, for example for driving to work.
If someone is named as an additional driver but in fact has main use of the car, this is called fronting, which is illegal. This often happens with parents and young drivers. Because new, inexperienced drivers face high premiums, a policy might instead be taken out with mum or dad as the main driver and the high-risk child as a named driver. But if the young driver in fact plans to have main use of the car, this is considered fraud.
On the other hand, if you share your vehicle with someone else in equal measure, speak to your insurance provider. You may need to take out a joint insurance policy, or else they can advise you on the next steps.