If you are hosting a party outdoors in the summer it is good to provide shade or sunscreen for your guests. The last thing you want is for one of your friends to end up with a bad burn and be miserable for the next few days. If you can host the part in an area where there is the option for shade.
Natural shade is the best choice. Not just since the idea of a shade tree is so appealing but it cools the area and those under it better. The loose pattern of the leaves allows heat to rise right through the tree canopy and not get trapped down by us ground dwellers. Now you do need to be careful of trees and some bugs like ticks but that is an issue almost anywhere. Also, try and keep food covered if you are keeping it in the shade under a tree.
A park with trees, under a structure with a tent. But if those are not an option then you will have to make arrangements to bring the sun protection with you.
You can pack umbrellas these can be purchased cheap at many stores and you can either get the kind with a base on it or one of the ones with sharpened end you can stick into the ground. Try to get enough so that at-least 20% of your gathering can be out of the shade. If the umbrellas won't work for you go for shade in a bottle.
Sun screen does not help reduce heat issues and resulting distress with heat stroke or dehydration but it will help keep away the burn. It should be re-applied every two hours on children. Adults can choose for them self but all to often they forget and get burned. Be a friend and remind your guests to lather up.
In reality sun screen should be worn even if shade is available but you knew that right?