sideliner

plural sideliners

1. One who stays on the sidelines; a spectator or nonparticipant.

   The result is more dissent because successful collective action sustains the involvement of old participants while convincing sideliners of the usefulness of protest and ensuring their future participation […]

   1936, American Sociological Review, volume 61, page 135

   1 more example

   It’s a bold statement (not to mention a cliché), but Mr. Cardiello, who created this high-energy workout to bring football to sideliners, isn’t afraid to be bold: he is a private trainer for N.F.L. players and former conditioning coach in the Arena Football League.

   2007 January 18, Abby Ellin, “Hard, Easy or Just Right?”, in New York Times

2. USA beekeeper, neither a hobbyist nor a fully-fledged commercial operator, for whom beekeeping is a secondary source of income.
3. One who sidelines or marginalizes something.