The szlachta ([ˈʂlaxta] ( listen), exonym: Nobility) was a legally privileged noble class with origins in the Kingdom of Poland. It gained considerable institutional privileges between 1333 and 1370 during the reign of King Casimir III the Great.:211 In 1413, following a series of tentative personal unions between the Grand Duchy of Lithuania and the Crown Kingdom of Poland, the existing Lithuanian nobility formally joined this class.:211 As the Polish-Lithuanian Commonwealth (1569–1795) evolved and expanded in territory, its membership grew to include the leaders of Ducal Prussia, Podolian and Ruthenian lands. 

The origins of the szlachta are shrouded in obscurity and mystery and have been the subject of a variety of theories.:207 Traditionally, its members were owners of landed property, often in the form of "manor farms" or so-called folwarks. The nobility negotiated substantial and increasing political and legal privileges for itself throughout its entire history until the decline of the Polish Commonwealth in the late 18th century.
Who else was added later?
the existing Lithuanian nobility