sideliner

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 […]

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.

A beekeeper, neither a hobbyist nor a fully-fledged commercial operator, for whom beekeeping is a secondary source of income.

One who sidelines or marginalizes something.