An additional insured is covered under the policy only for liability because of your actions. That is, if you do something and the additional insured is brought into the claim because of what you did, the policy coverage extends to the additional insured. The policy does not cover an additional insured for its own actions unrelated to work you do.
Your crew includes anyone who acts at your direction or collaborates with you on activities covered by your liability coverage. Adding a crew member adds them as an insured to your general liability coverage so they are covered for work they do for you.
While adding additional insureds to your general liability insurance protects the additional insured with respect to their relationship with you, adding crew protects you and your business in its day-to-day operations.