You have a bunch of guilds to choose from for a sentinel concept. Those I know of are currently:
We, the Thornglaive Sentinels, of course :
https://eu.forums.blizzard.com/en/wow/t/kaldorei-worgen-rp-thornglaive-sentinels-back-in-feathermoon-new-uniform-rules/191253
The Nightblades:
https://eu.forums.blizzard.com/en/wow/t/%E2%B8%B0%E2%98%BD-the-nightblade-sentinels-%E2%98%BE%E2%B8%B0-anu-kalimdor-%F0%9F%8C%99/19730
The Glaivewings:
https://eu.forums.blizzard.com/en/wow/t/arp-sentinel-cadre-glaivewings/122779
And Dirge, which are Black Moon army, I believe:
https://eu.forums.blizzard.com/en/wow/t/pcu-worgen-night-elf-rp-dirge-of-teldrassil/72859
Best check out this thread, too. There’s a guild list for night elf guilds there, public events and also communities, I believe. Also some night elf lore tidbits, especially for BfA lore.