Summon workers are the workers you hire for a certain job when you lack regular workers. You can find them in the first and next to the last tabs of the in-game Store. Also you can summon them right through the information window you receive when you want to launch a deal and there are no enough free hands in the village. Please, note that the guest workers will disappear as soon as they fulfill the job they were hired for! However, if you hired them to clean your island from small stones, grass and other obstacles, they will not leave until they clean everything you pointed out.