When you have an office you can appoint staff members to your facilities. You can either convert old players to staff or hire external staff members. This help category will explain this part of the game.
Staff is for advanced teams with several high level facilities. The advantaged they give are relatively small compared to upgrading the level of your facility (especially the early levels) and their wages can become very high. Appointing staff to low level facility may cause you to lose more money than you earn with them.