Some buildings must be staffed to operate. You have the option to hire your friends by clicking on the "Hire Friend" or "ask" button which brings up a menu that allows you to individually invite specific friends to work in that selected building. If they accept and all the spots are filled the building will be complete. This usually requires a little waiting depending on how fast your friends respond. It helps to have a lot of friends playing the game. A friend can work in more than one business. You also have the option to instantly hire a non-neighbor employee by using diamonds instead. It is usually five diamonds per employee.