People who are not directly involved in an activity or situation, but are watching or giving support from the outside.
Sideliners are usually not the main focus, but they can still have an important role.
Examples:
The sideliners cheered for their team.
The sideliners watched the game from the side.
The sideliners clapped and shouted for their friends.
The sideliners brought snacks to share.
The sideliners felt happy when their team won.
The sideliners waved flags and banners to show their support.