What does an additional insured endorsement do? Quite simply the endorsement adds another entity to your insurance policy. Lets say the following is what you are going through right now;
Property Management Company in charge of facility where you are holding a special event may require that their company and the building owner be added to your policy with regards to your special event. This is done by an endorsement to the insurance policy called, "additional insured endorsement". Anyone that enters a contract with a Facility to perform or utilize, will typically require that the Facility be named as an additional insured including an actual endorsement to the insurance policy. All places have their own rules and these are very different from venue to venue.
The other side of this is that when you contract with a vendor for service for your organization you should require that they add your entity to their policy as an additional insured with an actual endorsement to the insurance policy. The reason is that this entity can create a situation that you cannot control that causes damage to your guests and if he doesn't have insurance you will be held liable for the damages this professional causes. An example of this; A Disc Jockey is hired to perform at your special event and as you have gathered everyone around for some speaking. A loud Pop comes from the speakers and ruptures 3 guests ear drums. If you were added to the Disc Jockey's special event insurance policy by additional insured endorsement then you would be covered. This shouldn't cost the DJ enough to change his pricing structure and it is often no additional cost whatsoever.
Please be careful to review and utilize a special event insurance policy as Homeowners liability insurance will typically not meet these type of requirements.