Step 1. Locate the policy. Use the menu hamburger and select "Certificates and Docs"
Step 2. Choose the policy you want to add the additional insured to. Notice you can only add AI's to active and scheduled policies, you cannot add them to past policies.
Step 3. Select "+Additional Insured" to add an additional insured.
Step 4. Add the new Additional Insured to your account and touch "Done"
Step 5. Add the new Additional Insured to your policy by selecting it from the list below (see blue dots where I added two additional insureds) and touch "Add Selected Insureds" at the bottom.
Step 6. You're DONE!