MM-52600: [Shared Channels] Shared channels do not sync channel membership (#30976)

server/channels/app/shared_channel_test.go

@@ -16,6 +16,7 @@ func setupSharedChannels(tb testing.TB) *TestHelper {
 	return SetupConfig(tb, func(cfg *model.Config) {
 		*cfg.ConnectedWorkspacesSettings.EnableRemoteClusterService = true
 		*cfg.ConnectedWorkspacesSettings.EnableSharedChannels = true
+		cfg.FeatureFlags.EnableSharedChannelsMemberSync = true
 	})
 }